Fair game, lamprey's caught (4)
I believe the answer is:
prey
'fair game' is the definition.
(I know that fair game can be written as prey)
'lamprey's caught' is the wordplay.
'caught' says the answer is hidden in the clue (I've seen 'catches' mean this (the answer is caught in the clue)).
'PREY' is hidden in the letters of 'lampreys'.
(Other definitions for prey that I've seen before include "Quarry, fair game" , "Hunted creature" , "Animal hunted and killed for food" , "Hunter's quarry" , "Animal's quarry" .)