By GINIA BELLAFANTE

Until recently, she has been the Jackson defined by who she is not--not Michael, the surgically remastered chum of Macaulay Culkin, and not LaToya, the one short on talent but long on tabloid scandal. Today, though, JANET JACKSON, the singer with a hot album (janet.), the actress with a hit film (Poetic Justice), really is a star in her own right. And now she is about to receive one of the honors conferred only on those who achieve true star status: the unauthorized biography. In the proposal for the Janet expose, J. Randy Taraborrelli (author of Michael Jackson and Diana Ross bios) alleges not only that she has undergone numerous cosmetic surgeries (including fat removal), but also that she is secretly married. Another claim of Taraborrelli's is that at 15 Janet helped her mother "mercilessly" beat up a woman suspected of having an affair with Janet's father. The reporter promises plenty of other scandalous revelations too. A spokesman for the singer says the allegations are "absolutely untrue."