Unconvincingly feeble (4)
I believe the answer is:
lame
'unconvincingly feeble' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lame that I've seen before include "Weak and unconvincing like injured foot" , "Unable to walk without difficulty" , "Imperfect - material" , "Unable to walk properly" , "Weak kind of excuse for halting walk" .)