Involuntary reaction (4-4)
I believe the answer is:
knee-jerk
'involuntary reaction' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for knee-jerk that I've seen before include "(Of a reaction) automatic and unthinking" , "(Of a response) automatic and unthinking" , "Sudden and ill-considered" , "Involuntary or hasty (reaction)" , "Thoughtlessly spontaneous" .)