A fine honest business (6)
I believe the answer is:
affair
'business' is the definition.
(thesaurus)
'a fine honest' is the wordplay.
'a fine' becomes 'af' (I've seen this in other clues).
'honest' becomes 'fair' ('fair' can be similar in meaning to 'honest').
'af'+'fair'='AFFAIR'
(Other definitions for affair that I've seen before include "Secret liaison" , "Event; fling" , "Romantic relationship, perhaps illicit" , "Secret romance" , "Amorous liaison" .)