To provide sexual favors in exchange for musical production. This can and usually does encompass many forms of sexual gratification as payment for the vast kinds of technical direction needed to create a song or "track".
Record Producer #1: Whaddup dawg, I'm crazy broke right now.
Record Producer #2: Wait didn't you just finish working on that new Joss Stone
project over at EMI?
Record Producer #1: Yeah but she don't be payin' with cash. She likes Fuckin' for tracks.