Behave badly at university after play (3,2)
I believe the answer is:
act up
'behave badly' is the definition.
(acting up is a kind of behaving badly)
'at university after play' is the wordplay.
'at university' becomes 'up' (slang term meaning at university).
'after' means one lot of letters go next to another.
'play' becomes 'act' (synonyms).
'up' put after 'act' is 'ACT UP'.
(Other definitions for act up that I've seen before include "malfunction" , "Behave badly, cause trouble - like thespian?" , "take part in heightened drama?" , "Misbehave - carry on" , "go wrong" .)