Main troublemaker (10)
I believe the answer is:
ringleader
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">main troublemaker</span>' is the definition.<br/ ><span class="explanation-format__explanation">(eg the ringleader of a crime)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ringleader that I've seen before include "firebrand" , "Main villain" , "Chief mischief maker" , "Chief troublemaker" , "Mastermind" .)