Rue losing first bird? (5)
I believe the answer is:
egret
'bird?' is the definition.
(egret is a kind of bird)
'rue losing first' is the wordplay.
'rue' becomes 'regret' (synonyms).
'losing first' means to remove the first letter.
'regret' with its initial letter removed is 'EGRET'.
(Other definitions for egret that I've seen before include "Heron-like wader" , "Lesser white heron" , "one with plume" , "Wading bird like a heron" , "Stork-like bird" .)