Criminally aid (4)
I believe the answer is:
abet
'criminally aid' 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 "Help in wrongdoing" , "Assist (a criminal)" , "Incite or encourage" , "Beat up by way of help" , "Assist (in wrongdoing)" .)