Misbehave, or pretend to be high (3,2)
I believe the answer is:
act up
'misbehave' is the definition.
(acting up is a kind of misbehaving)
'pretend to be high' is the wordplay.
'pretend to be' becomes 'act' (I've seen this in another clue).
'high' becomes 'up' (associated in meaning).
'act'+'up'='ACT UP'
'or' acts as a link.
(Other definitions for act up that I've seen before include "Give trouble, make a fuss" , "Informally, behave badly or cause trouble" , "Behave badly, cause trouble - like thespian?" , "Mess about" , "Behave like a brat" .)