Behave badly in a time-consuming contest (3,2)
I believe the answer is:
act up
'behave badly' is the definition.
(act up can mean to misbehave)
'a time-consuming contest' is the wordplay.
'time' becomes 't' (abbreviation).
'consuming' is an insertion indicator (consume can mean to bring in or absorb).
'contest' becomes 'cup' (a sporting contest eg the FA Cup).
't' inserted within 'cup' is 'ctup'.
'a'+'ctup'='ACT UP'
'in' is the link.
(Other definitions for act up that I've seen before include "Misbehave - carry on" , "Become unpleasant" , "Behave badly, give trouble" , "Mess about" , "Give trouble, make a fuss" .)