Which women's accessory? (4)
I believe the answer is:
what
'which' is the definition.
(I've seen this in other clues)
'women's accessory?' is the wordplay.
'women' becomes 'w' (standard abbreviation).
'accessory?' becomes 'hat' (I've seen this before).
'w'+'hat'='WHAT'
(Other definitions for what that I've seen before include "I didn't hear" , "Interrogatory word" , "That which - come again!" , "I beg your pardon" , "Word used in a question" .)