Be an accomplice to (4)
I believe the answer is:
abet
'be an accomplice to' 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 "Beat up by way of help" , "Aid in offence" , "Aid (a criminal)" , "Aid - connive with" , "Criminally endorse" .)