Fish eater's very big quarry (6)
I believe the answer is:
osprey
'fish eater's' is the definition.
(I've seen this in another clue)
'very big quarry' is the wordplay.
'very big' becomes 'os' (abbreviation for outsize).
'quarry' becomes 'prey' (synonyms).
'os'+'prey'='OSPREY'
(Other definitions for osprey that I've seen before include "Fish-eater" , "Large bird that preys on fish in inland waters" , "Spy roe (anag)" , "Marine bird of prey" , "Piscivorous creature" .)