Misbehaves when outside court, and in court! (4,2)
I believe the answer is:
acts up
'misbehaves' is the definition.
(acting up is a kind of misbehaving)
'when outside court and in court' is the wordplay.
'when' becomes 'as'.
'outside' means one lot of letters goes inside another.
'court' becomes 'ct'.
'and' means one lot of letters go next to another.
'in court' becomes 'up'.
'as' enclosing 'ct' is 'acts'.
'acts'+'up'='ACTS UP'
(Other definitions for acts up that I've seen before include "Malfunctions" , "Fails to function properly or behaves badly" , "Behaves badly (colloq.)" , "Causes trouble or misbehaves" , "Carries on" .)