Young bird lacking the knowledge to be fashionable (4)
I believe the answer is:
chic
'to be fashionable' is the definition.
(chic means stylish or fashionable)
'young bird lacking' is the wordplay.
'young bird' becomes 'chick' (I've seen this in another clue).
'lacking' means to remove the last letter (the word is incomplete or lacking).
'chick' with its last letter taken away is 'CHIC'.
'the knowledge' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for chic that I've seen before include "Elegantly stylish" , "Unwanted sounds" , "Trendy" , "Fashionable good taste" , "Fashionably elegant" .)