Man wrong about Missouri (5)
I believe the answer is:
simon
'man' is the definition.
(I've seen this before)
'wrong about missouri' is the wordplay.
'wrong' becomes 'sin' (I've seen this before).
'about' is an insertion indicator.
'missouri' becomes 'mo'.
'sin' placed around 'mo' is 'SIMON'.
(Other definitions for simon that I've seen before include "See 1" , "Harris and Coveney" , "Simple chap" , "Fellow" , "One of Christ's apostles" .)