Young bird, not completely fashionable (4)
I believe the answer is:
chic
'fashionable' is the definition.
(chic means stylish or fashionable)
'young bird not completely' is the wordplay.
'young bird' becomes 'chick' (I've seen this before).
'not completely' means to remove the last letter (the word is incomplete).
'chick' with its final letter removed is 'CHIC'.
(Other definitions for chic that I've seen before include "Stylish and elegant - in France" , "Elegantly fashionable" , "French elegance" , "Modish" , "Elegant and stylishly fashionable" .)