Be mischievous cat playing in high place (3,2)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
act up
(Other definitions for act up that I've seen before include "create fireworks display?" , "Informally, behave badly or cause trouble" , "Behave badly, give trouble" , "take part in heightened drama?" , "Behave like a brat" .)