Answer, reaction (8)
I believe the answer is:
response
I believe this clue is a double definition.
'answer' is the first definition.
(I know that answer can be written as response)
'reaction' is the second definition.
(I know that reaction can be written as response)
(Other definitions for response that I've seen before include "'Answer, reply (8)'" , "counter" , "Reaction, reply" , "Preens so by way of answer" , "Press one for a reply" .)