Assist (a criminal) (4)
I believe the answer is:
abet
'assist a criminal' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for abet that I've seen before include "Crookedly endorse" , "Incite (to commit an offence)" , "Beat up by way of help" , "Assist in some nefarious activity" , "Aid in offence" .)