Team leader entering a competition to misbehave (3,2)
I believe the answer is:
act up
'misbehave' is the definition.
(acting up is a kind of misbehaving)
'team leader entering a competition' is the wordplay.
'leader' says to take the initial letters.
'entering' indicates putting letters inside.
'competition' becomes 'cup' (eg the FA Cup).
The first letter of 'team' is 't'.
'a'+'cup'='acup'
't' inserted into 'acup' is 'ACT UP'.
'to' acts as a link.
(Other definitions for act up that I've seen before include "Informally, behave badly or cause trouble" , "malfunction" , "Become unpleasant" , "Behave badly, give trouble" , "go wrong" .)