When Main Source dropped their first single, ''Looking at the Front Door,'' the hip-hop community went wild. It seemed as if every rap deejay was playing the song on the radio or at parties. The tune, which went to the top of the rap-singles charts, is a swinging look at a crumbling relationship, punctuated by a sampled chorus of sweet, melodic R&B. With a smooth baritone, 18-year-old rapper Large Professor touches upon a sadness and humility rare in the rap world: ''We fight every night/Now that's not kosher/I reminisce with bliss of when we was closer/And wake up to be greeted by an argument again.'' Elsewhere on Breaking Atoms, their full-length debut, Main Source may not break much new ground, but they offer a clever, quietly seductive collection in which the bass and drum tracks casually strut instead of stomp, and the sparse samples of guitar and horns allow the Large Professor's voice to take center stage. That's clearly where he belongs.