Aid in mischief (4)
I believe the answer is:
abet
'aid in mischief' 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 "Aid and help, perhaps in crime" , "Aid as in committing crime" , "Help a criminal" , "Aid - connive with" , "Give aid to" .)