Idiot with success concealing dreadful flaw (7)
I believe the answer is:
half-wit
'idiot' is the definition.
(I've seen this before)
'success concealing dreadful flaw' is the wordplay.
'success' becomes 'hit' (hit is a kind of success**).
'concealing' means one lot of letters goes inside another.
'dreadful' indicates anagramming the letters.
'flaw' anagrammed gives 'alfw'.
'hit' going around 'alfw' is 'HALF-WIT'.
'with' is the link.
(Other definitions for half-wit that I've seen before include "One only 50% all there?" , "Idiot" , "Fool" , "Stupid person" , "Nincompoop" .)