Causes trouble (4,2)
I believe the answer is:
acts up
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">causes trouble</span>' is the definition.<br/ ><span class="explanation-format__explanation">(act up can mean to misbehave)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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.)" .)