Good person bad at sin (5)
I believe the answer is:
saint
'good person' is the definition.
(saint is a kind of good person)
'bad at sin' is the wordplay.
'bad' indicates anagramming the letters.
'at'+'sin'='atsin'
'atsin' with letters rearranged gives 'SAINT'.
(Other definitions for saint that I've seen before include "Canonised person" , "Good woman" , "paragon!" , "Holy man" , "Beatified person" .)