Not just dishonest (6)
I believe the answer is:
unfair
'dishonest' is the definition.
(thesaurus)
'not just' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'just' becomes 'fair' ('fair' is associated in meaning with 'just'**).
'un'+'fair'='UNFAIR'
(Other definitions for unfair that I've seen before include "Swizz!" , "not right" , "deceitful" , "Weighted" , "Unjust, partial" .)