Son isn't a good person (5)

I believe the answer is:

saint

'a good person' is the definition.
(saint is a kind of good person)

'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!" , "Good woman" , "eg John, Paul or George" , "Holy person" , "Beatified person" .)

I've seen this clue in The Times.

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