Son is not commonly a good person (5)
I believe the answer is:
saint
'a good person' is the definition.
(I know that saint is a type of good person)
'son is not commonly' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'is not commonly' becomes 'aint' (non-standard version of 'isn't' or 'aren't').
's'+'aint'='SAINT'
(Other definitions for saint that I've seen before include "eg Pancras" , "See 6" , "Scotland's oldest university" , "Very virtuous person" , "Holy person" .)