Weak — as a sucker? (9)
I believe the answer is:
toothless
I believe this clue is a double definition.
'weak' is the first definition.
(similar in meaning)
'as a sucker?' is the second definition.
(someone without teeth might suck)
(Other definitions for toothless that I've seen before include "Lacking in force" , "Deprived of power" , "Lacking power" , "Unable to chew; ineffective (sanction)" .)