Making mischief (2,2,2,4)
I believe the answer is:
up to no good
'making mischief' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for up to no good that I've seen before include "Making mischief" , "Doing something wrong" , "Misbehaving" , "Doing something mischievous" , "Scheming to do wrong(2.2.2.4)" .)