Wrong to assume good character (4)
I believe the answer is:
sign
'character' is the definition.
(thesaurus)
'wrong to assume good' is the wordplay.
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'to assume' is an insertion indicator.
'good' becomes 'g' (abbreviation).
'sin' placed around 'g' is 'SIGN'.
(Other definitions for sign that I've seen before include "Evidence" , "Token - symbol - omen - write one's name" , "Indication; symbol" , "Put one's name to a document" , "information board?" .)