Sucker beat target! (7)
I believe the answer is:
lamprey
'sucker' is the definition.
(aquatic animals with sucker mouths)
'beat target' is the wordplay.
'beat' becomes 'lam' (I've seen this in other clues).
'target' becomes 'prey' (synonyms).
'lam'+'prey'='LAMPREY'
(Other definitions for lamprey that I've seen before include "Swimmer" , "Eel-like vertebrate with a sucker mouth and horny teeth" , "sucker in the main" , "sucker for stones" , "seafood" .)