Nincompoop's bad flaw concealed by success (4-3)
I believe the answer is:
half-wit
'nincompoop's' is the definition.
(I've seen this before)
'bad flaw concealed by success' is the wordplay.
'bad' indicates anagramming the letters.
'concealed by' is an insertion indicator.
'success' becomes 'hit' (hit is a kind of success).
'flaw' with letters rearranged gives 'alfw'.
'alfw' going within 'hit' is 'HALF-WIT'.
(Other definitions for half-wit that I've seen before include "Nincompoop" , "Fool" , "One only 50% all there?" , "Hit flaw (anag.)" , "Idiot" .)