BORN CHRISTMAS Day 1948, backup singer Merry Clayton has been lending her skills to pop musicians for most of her 64 years.

Usually she's the "last person, along with the string players," to add their flavor to a studio recording session. "That's why they call it 'sweetening,' " she explained in a recent chat.

More often than not, she's the least recognized, least thanked contributor, though Clayton's blow- torched, highly emotive vocals made a major difference on massive hits such as Lynyrd Skynyrd's "Sweet Home Alabama," Carole King's "Way Over Yonder," Ray Charles' "Let's Go Get Stoned" and Joe Cocker's "Feelin' Alright."

And, yes, that's Clayton sending shivers up our collective spine with the moans of "Rape! Murder! It's just a shot away!" on the Rolling Stones megahit "Gimme Shelter."

In all instances, Clayton's gift is taking something good and goosing it 'til it's great, adding the fire and brimstone of a gospel-trained wailer, bringing a stamp of authentic soul.

The studio and touring work has always been its own reward, she says. "You work with everyone you love and respect and you make good money."

But you better believe Clayton is feeling more than all right these days.

Shine a light

"I'm so good, I can't stand myself," she joked, during an interview promoting the terrific new film documentary "20 Feet From Stardom" (already being touted as an Academy Award contender) and also the aptly timed album "The Best of Merry Clayton."

The film throws the spotlight on the second line of concert and studio-singing talents - mostly church-bred and women of color - who've added character and conviction to popular music "and are usually better singers than the person up front," opines the film's director Morgan Neville.

Along with Clayton, the doc gives major props to Darlene Love, Lisa Fischer, Judith Hill, Claudia Lennear and Tata Vega.

Clayton goes "way back with Darlene, to the Blossoms" - historically the first and most successful of female backing groups.

"Darlene became my mentor when I was 14 or 15 and just getting started working with Bobby Darin," Clayton recalled.

And now all five featured femmes have grown close making the film - which packs new collaborative performances as well as some oldies.

"We're even talking about going out on tour together, when our schedules are cleared," Clayton said.

Fischer is busiest of the bunch, on the road with the Rolling Stones. Love is the most high profile - with the best backstory thanks to her slavish contract with the notorious Phil Spector and subsequent screw-over by Philadelphia International Records (which sold her follow-up contract back to Spector). Hill is the youngest, an up-and-comer, prominently working with Michael Jackson at the time of his death.

Now and then

Clayton had to think a minute before responding to our question "What have you been doing lately?" (A low-key jazz project.) Her new "best-of" album tracks solos from 1969 to 1975, mostly cover sessions ("Southern Man," "Suspicious Minds," "A Song for You") grooming Clayton as the "next" Aretha Franklin.

"The music business has changed so much," she said. "Nowadays, people record in their basement studios and use just anybody - their cousin, the kid down the block - as their backup singers, then fix the bad singing 'in the mix' with Pro Tools. I used to hang out at A&M Records [whose recently deceased president Gil Friesen kick-started '20 Feet From Stardom'] and it was a really social, productive scene. I might be working on my own album, or just sitting on the bench in the hall, and could fall into record projects with whoever was working there that day. That's how I got onto the 'Tapestry' album for instance - Carole King just pulling me into the studio with the line 'Got a minute? I need your help.' "

Just a shot away

Her most famous recording with the Rolling Stones was almost as spontaneous.

Clayton's producer-buddy Jack Nitzsche roused her from bed at "11:30 or midnight" with a phone call that he had "this British band in town that was having trouble finding someone to come in and sing a part and was I available. I was four-and-a-half months pregnant, but my husband [jazz saxophonist Curtis Amy] convinced me I should do it. I kept my pink silk pajamas and fuzzy slippers on, put a cream Chanel scarf over my rollers, threw on my mink coat and went down to the studio, where I met Keith and then Mick. They showed me the part, explained the story of the song, why I should be singing 'rape! murder!' and then I sang it two, no, three times. I was so heavy in the midsection I had to sing sitting down on a stool. That's all it took, that's all I could manage. I had to go home. But from watching [Richards and Jagger] jumping up and down in the control room, I knew we had something."

The good news is that Clayton became a music industry sensation, the "queen of the rock backup singers" from working on that session. And while she originally agreed to do the "Gimme Shelter" session just for a "triple time" hourly union rate, the featured soloist wound up negotiating an ongoing-slice of the performance royalties that's still keeping her in fuzzy slippers.

The bad news is that Clayton reportedly miscarried - maybe from the stress and strain of the session.

The gospel tradition

Clayton was never awed by music celebrities. Her father was the music-minded minister of New Orleans' New Bethel Baptist Church.

"Everybody that was anybody would come and hang in my dad's church, participate in the service. Sam Cooke . . . Lou Rawls . . . Della Reese, who's my godmother. And many mornings, I'd find myself sitting on a pew next to Mahalia Jackson. I'd go to sleep on her arm and put my feet up on Linda Hopkins. Everything Mahalia would sing, I'd mimic. And then they started calling me 'Little Haley' when I was about 6 or 7 years old."

Another big role model "and surrogate father" was Ray Charles, whom she joined as a backing Raelet after an invite from "my great friend Billy Preston, who was then in the band. I joined Ray on March 23, 1966, and stayed with him three-and-a-half years. I remember that date clearly because it was also the day I met my husband-to-be [Amy], a brilliant conductor and arranger and musician. We stayed married 32 years, until his death."

So Clayton never fell prey to the wiles of legendary lothario Charles?

"It used to be said you had to 'Let Ray' if you wanted to be a Raelet. Well honey, I was too busy letting the conductor."