Be an accomplice (4)
I believe the answer is:
abet
'be an accomplice' 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 "Incite (to commit an offence)" , "Support, aid" , "Help a criminal" , "Assist illegally" , "Beat up by way of help" .)