Grass is criminal (4)
I believe the answer is:
bent
I believe this is a double definition.
'grass' is the first definition.
(I've seen this before)
'is criminal' is the second definition.
(I've seen this before)
(Other definitions for bent that I've seen before include "Slang for dishonest or crooked" , "Dishonest; grass" , "Special skill" , "Natural skill to be slangily dishonest?" , "on the fiddle" .)