Simpleton finds bad flaw in strike
I believe the answer is:
halfwit
'simpleton' is the definition.
(half-wit is a kind of simpleton)
'bad flaw in strike' is the wordplay.
'bad' indicates an anagram.
'in' means one lot of letters goes inside another.
'strike' becomes 'hit' (synonyms).
'flaw' is an anagram of 'alfw'.
'alfw' inserted inside 'hit' is 'HALFWIT'.
'finds' acts as a link.
(Other definitions for halfwit that I've seen before include "Mug" , "Charlie?" , "One only 50% all there?" , "Idiot" , "dunce?" .)