Separate crest for game bird (9)
I believe the answer is:
partridge
'for game bird' is the definition.
(type of game bird)
'separate crest' is the wordplay.
'separate' becomes 'part' (both can mean to split or break apart).
'crest' becomes 'ridge' (geographical term).
'part'+'ridge'='PARTRIDGE'
(Other definitions for partridge that I've seen before include "Flighty type" , "early Christmas present" , "sort of quail" , "Widespread bird" , "1st" .)