Trendy young bird dumps knight (4)
I believe the answer is:
chic
'trendy' is the definition.
(I've seen this before)
'young bird dumps knight' is the wordplay.
'young bird' becomes 'chick' (I've seen this in another clue).
'dumps' indicates named letters should be taken away.
'knight' becomes 'k'.
'chick' with 'k' taken out is 'CHIC'.
(Other definitions for chic that I've seen before include "In vogue" , "style?" , "cool" , "French elegance" , "Fashionable good taste" .)