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