Sucker punch victim (7)
I believe the answer is:
lamprey
'sucker' is the definition.
(aquatic animals with sucker mouths)
'punch victim' is the wordplay.
'punch' becomes 'lam' (lam can mean to hit).
'victim' becomes 'prey' (prey is a kind of victim).
'lam'+'prey'='LAMPREY'
(Other definitions for lamprey that I've seen before include "Eel-like vertebrate with a sucker mouth and horny teeth" , "sucker in the main" , "Freshwater eel-like fish" , "Marine carnivore" , "Eel-like creature - play REM (anag)" .)