Crossword Genius

Trendy bright female, at heart, is a poser (6)

Ross

I believe the answer is:

enigma

'poser' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for enigma that I've seen before include "Puzzling person/thing" , "'Mystery, riddle (6)'" , "Elgar Variations nickname" , "WW two Ger. code device; puzzle" , "cryptic puzzle!" .)

I've seen this clue in The Guardian.
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