Willing bird (4)
I believe the answer is:
game
This clue is a double definition.
'willing' is the first definition.
(game can mean willing to do something)
'bird' is the second definition.
(game bird is a kind of bird)
(Other definitions for game that I've seen before include "Lark" , "Animals hunted for food" , "See 3" , "Pastime; brave" , "Final declaration of tennis result - . . . ., set and match" .)