Ball of fire (4)
I believe the answer is:
doer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ball of fire</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for doer that I've seen before include "Active type" , "Martha?" , "Achiever" , "Active person" , "Person who acts" .)