Hit and kill fish (7)
I believe the answer is:
lamprey
'fish' is the definition.
(I've seen this before)
'hit and kill' is the wordplay.
'hit' becomes 'lam' (to 'lam' means to hit hard).
'and' means one lot of letters go next to another.
'kill' becomes 'prey' (a 'kill' is a hunter's prey).
'lam'+'prey'='LAMPREY'
(Other definitions for lamprey that I've seen before include "sucker for stones" , "Eel-like creature - play REM (anag)" , "Long fish" , "Marine carnivore" , "Fish-like vertebrate" .)