Bad one if converted in good faith (4,4)
I believe the answer is:
bona fide
'in good faith' is the definition.
(I've seen this in another clue)
'bad one if converted' is the wordplay.
'converted' indicates anagramming the letters.
'bad'+'one'+'if'='badoneif'
'badoneif' with letters rearranged gives 'BONA FIDE'.
(Other definitions for bona fide that I've seen before include "Authentic" , "(Proof of being) genuine" , "it's on the level" , "Real, genuine (Latin)" , "honest - with 12a" .)