Natural aptitude for being dishonest (4)
I believe the answer is:
bent
I believe this clue is a double definition.
'natural aptitude' is the first definition.
(I've seen this before)
'being dishonest' is the second definition.
(thesaurus)
'for' is the link.
(Other definitions for bent that I've seen before include "Dishonest; angled" , "passion" , "Angled; corrupt" , "Natural talent" , "Crooked - inclination" .)