Behave badly, as a court's risen (3,2)
I believe the answer is:
act up
'behave badly' is the definition.
(acting up is a kind of behaving badly)
'a court's risen' is the wordplay.
'court' becomes 'ct'.
'risen' becomes 'up' ('up' can be similar in meaning to 'risen').
'a'+'ct'+'up'='ACT UP'
'as' is the link.
(Other definitions for act up that I've seen before include "Give trouble, make a fuss" , "go wrong" , "Behave badly (colloq.)" , "Behave like a brat" , "Misbehave - carry on" .)