Pained reaction (5)
I believe the answer is:
wince
'pained reaction' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wince that I've seen before include "React involuntarily from pain or embarrassment" , "Visibly react to pain" , "Start back in pain" , "Involuntary grimace of pain" , "Make face showing pain" .)