Propensity to become dishonest (4)
I believe the answer is:
bent
I believe this clue is a double definition.
'propensity' is the first definition.
(bent can mean a propensity or tendency)
'dishonest' is the second definition.
(bent can mean corrupt)
'to become' acts as a link.
(Other definitions for bent that I've seen before include "Curved, crooked" , "Angled; corrupt" , "Slang for dishonest or crooked" , "Dishonest; angled" , "Dishonest; curved" .)