Criminal is grass (4)
I believe the answer is:
bent
This clue is a double definition.
'criminal' is the first definition.
(I've seen this before)
'grass' is the second definition.
(I've seen this before)
'is' acts as a link.
(Other definitions for bent that I've seen before include "Sharply curved, grass" , "Crooked - penchant" , "Curved, crooked" , "Twisted or determined" , "Flexed -- penchant" .)