Automatic response (6)
I believe the answer is:
reflex
'automatic response' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for reflex that I've seen before include "This action is an automatic response to stimulus" , "Unthinking response" , "Bent backwards" , "Involuntary reaction to stimulus" , "Involuntary response" .)