Behave badly at university after work (3,2)
I believe the answer is:
act up
'behave badly' is the definition.
(act up can mean to misbehave)
'at university after work' is the wordplay.
'at university' becomes 'up' (slang term meaning at university).
'after' means one lot of letters go next to another.
'work' becomes 'act' (synonyms).
'up' after 'act' is 'ACT UP'.
(Other definitions for act up that I've seen before include "Behave badly, give trouble" , "Give trouble, make a fuss" , "Behave badly (colloq.)" , "Misbehave - carry on" , "malfunction" .)