Endlessly weak victim is sucker (7)
I believe the answer is:
lamprey
'is sucker' is the definition.
(aquatic animals with sucker mouths)
'endlessly weak victim' is the wordplay.
'endlessly' means to remove the last letter.
'weak' becomes 'lame' (similar in meaning).
'victim' becomes 'prey' (prey is a kind of victim).
'lame' with its final letter removed is 'lam'.
'lam'+'prey'='LAMPREY'
(Other definitions for lamprey that I've seen before include "Eel-like vertebrate with a sucker mouth and horny teeth" , "Marine carnivore" , "sucker in the main" , "Fish with sucker mouth" , "Eel-like creature - play REM (anag)" .)