Involuntary response (6)
I believe the answer is:
reflex
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">involuntary response</span>' is the definition.<br/ ><span class="explanation-format__explanation">(involuntary bodily reaction)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for reflex that I've seen before include "This action is an automatic response to stimulus" , "Unconscious response to stimulus" , "Unthinking response" , "Involuntary (action)" , "Knee-jerk" .)