Son isn’t a very good person (5)
I believe the answer is:
saint
'a very good person' is the definition.
(I've seen this in another clue)
'son isn't' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'isn't' becomes 'aint' (non-standard version of 'isn't').
's'+'aint'='SAINT'
(Other definitions for saint that I've seen before include "Beatified person" , "eg John, Paul or George" , "Canonised person" , "Very virtuous person" , "Holy man" .)