。
She's not a poet but a novelist.
Literal
She [topic-は] poet not, novelist is.
The pattern Xではなく + Y is a contrastive negation — 'not X but Y.' ~ではなく is the conjunctive form of the negative copula ではない, used here to deny one identity and affirm another. The casual contraction じゃなく works the same way. This frame is the standard way to correct or replace a category — useful whenever you need to say 'it's actually Y, not X.' 詩人 ('poet') and 小説家 ('novelist') are paired here as kindred but distinct literary professions.