Trendy young bird sheds kilo (4)
I believe the answer is:
chic
'trendy' is the definition.
(I've seen this in another clue)
'young bird sheds kilo' is the wordplay.
'young bird' becomes 'chick' (I've seen this before).
'sheds' is a deletion indicator.
'kilo' becomes 'k' (phonetic alphabet: alpha, bravo, charlie etc.).
'chick' with 'k' removed is 'CHIC'.
(Other definitions for chic that I've seen before include "French elegance" , "A la mode" , "Stylishness, elegance" , "Fashionable good taste" , "In vogue" .)