Impetus, spur (8)
I believe the answer is:
stimulus
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">impetus</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">spur</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for stimulus that I've seen before include "That which produces a response" , "Fillip" , "Spur or catalyst for action" , "Something provoking a reaction" , "Trigger" .)