Tense son in a competition misbehaves (4,2)
I believe the answer is:
acts up
'misbehaves' is the definition.
(acting up is a kind of misbehaving)
'tense son in a competition' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'son' becomes 's'.
'in' is an insertion indicator.
'competition' becomes 'cup' (I've seen this before).
't'+'s'='ts'
'a'+'cup'='acup'
'ts' put within 'acup' is 'ACTS UP'.
(Other definitions for acts up that I've seen before include "Behaves in a troublesome way" , "Malfunctions" , "Misbehaves badly" , "Carries on" , "Behaves badly (colloq.)" .)