Involuntary response (6)
I believe the answer is:
reflex
'involuntary response' is the definition.
(involuntary bodily reaction)
This is all the clue.
(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" .)