Aid a criminal (4)
I believe the answer is:
abet
'aid a criminal' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abet that I've seen before include "Encourage (wrongdoing)" , "Assist in some nefarious activity" , "Aid (in wrongdoing)" , "Encourage (offence)" , "Aid and assist" .)