Bilk (3)
I believe the answer is:
con
'bilk' is the definition.
(I know that bilk is a more specific form of the action con)
This is all the clue.
(Other definitions for con that I've seen before include "Swindle, deception" , "Tory" , "Deceive; disadvantage" , "Opposed" , "Swindle in draconian way" .)