Son isn’t a good man (5)
I believe the answer is:
saint
'a good man' is the definition.
'son isn't' is the wordplay.
'son' becomes 's'.
'isn't' becomes 'aint' (I've seen this before).
's'+'aint'='SAINT'
(Other definitions for saint that I've seen before include "paragon!" , "Canonised person" , "Holy person" , "eg Paul, Peter, Valentine" , "See 6" .)