Behave badly (3,2)
I believe the answer is:
act up
'behave badly' is the definition.
(act up can mean to misbehave)
This is the entire clue.
(Other definitions for act up that I've seen before include "Behave like a brat" , "take part in heightened drama?" , "Give trouble, make a fuss" , "create fireworks display?" , "Become unpleasant" .)