Causes trouble (4,2)
I believe the answer is:
acts up
'causes trouble' is the definition.
(act up can mean to misbehave)
This is the entire clue.
(Other definitions for acts up that I've seen before include "Carries on" , "Behaves in a troublesome way" , "Fails to function properly or behaves badly" , "Misbehaves badly" , "Behaves badly (colloq.)" .)