Assist illegally (4)
I believe the answer is:
abet
'assist illegally' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for abet that I've seen before include "Incite or encourage" , "Aid and help, perhaps in crime" , "Aid as in committing crime" , "Crookedly endorse" , "Assist wrongly!" .)