Game bird is all prepared (9)
I believe the answer is:
billiards
'game' is the definition.
(billiards is a kind of game)
'bird is all prepared' is the wordplay.
'prepared' indicates an anagram.
'bird'+'is'+'all'='birdisall'
'birdisall' is an anagram of 'BILLIARDS'.
(Other definitions for billiards that I've seen before include "Game with three balls" , "Game played with balls on table with pockets" , "Table game at which Mary Queen of Scots excelled" , "Ball game" , "Indoor sport" .)