Aptitude to be crooked? (4)
I believe the answer is:
bent
This clue is a double definition.
'aptitude' is the first definition.
(bent is a kind of aptitude)
'be crooked?' is the second definition.
(both can mean corrupt)
'to' acts as a link.
(Other definitions for bent that I've seen before include "Slang for dishonest or crooked" , "passion" , "Natural aptitude" , "Crooked - tendency" , "Twisted - corrupt" .)