Sucker repeatedly hit target (7)
I believe the answer is:
lamprey
'sucker' is the definition.
(aquatic animals with sucker mouths)
'repeatedly hit target' is the wordplay.
'repeatedly hit' becomes 'lam' (lam can mean to hit violently).
'target' becomes 'prey' (synonyms).
'lam'+'prey'='LAMPREY'
(Other definitions for lamprey that I've seen before include "Fish-like vertebrate" , "sucker in the main" , "Eel-like animal" , "Eel-like bloodsucker" , "seafood" .)