Kind of reaction or instinct (3)
I believe the answer is:
gut
I believe this is a double definition.
'kind of reaction' is the first definition.
(I've seen this in another clue)
'instinct' is the second definition.
(Other definitions for gut that I've seen before include "Intestine, or the stomach" , "corporation" , "restricted passage" , "Internal channel" , "Food processor" .)