Sack; stimulate (4)
I believe the answer is:
fire
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">sack</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that sack can be written as fire)</span><br/ ><br/ >'<span class="explanation-format__original">stimulate</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(fire or fire up can mean to stimulate)</span><br/ ><br/ ></span>
(Other definitions for fire that I've seen before include "... or combustion" , "Trigger" , "Weaponry" , "Dismiss; bake in a kiln" , "Blaze - discharge" .)