Respond to a stimulus (5)
I believe the answer is:
react
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">respond to a stimulus</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for react that I've seen before include "Acknowledge" , "provide an answer" , "Respond emotionally" , "Respond to something" , "Show opposition" .)