Weak, unconvincing (4)
I believe the answer is:
lame
This clue is a double definition.
'weak' is the first definition.
(similar in meaning)
'unconvincing' is the second definition.
(I've seen this in another clue)
(Other definitions for lame that I've seen before include "Disabled in the feet or legs" , "Having impaired walking ability" , "Halt" , "Unable to walk without difficulty" , "Disabled in leg or foot" .)