Bad, if one turns out to be genuine (4,4)
I believe the answer is:
bona fide
'be genuine' is the definition.
('bona fide' can be similar in meaning to 'genuine')
'bad if one turns out' is the wordplay.
'turns out' indicates anagramming the letters.
'bad'+'if'+'one'='badifone'
'badifone' anagrammed gives 'BONA FIDE'.
'to' is the link.
(Other definitions for bona fide that I've seen before include "Real, genuine (Latin)" , "it's on the level" , "Genuine, without fraud - in Latin" , "honest - with 12a" , "Genuine or in good faith" .)