Very large game bird (6)
I believe the answer is:
osprey
'bird' is the definition.
(I know that osprey is a bird)
'very large game' is the wordplay.
'very large' becomes 'os' (abbreviation for oversize).
'game' becomes 'prey' (both can mean animals which are hunted).
'os'+'prey'='OSPREY'
(Other definitions for osprey that I've seen before include "Fish-eating raptor" , "Spy roe (anag)" , "I grab fish" , "predatory bird" , "Large bird that preys on fish in inland waters" .)