Reaction (8)
'reaction' is the definition.
(I know that reaction can be written as response)
This is the entire clue.
(Other definitions for response that I've seen before include "bid by partner" , "Acknowledgement" , "Reply, rejoinder" , "Reaction, reply" , "Answer - reaction" .)
(Other definitions for allergic that I've seen before include "Responding badly (to something)" , "Responding badly to certain substances" , "Hypersensitive (to certain substances)" , "Having an adverse reaction (to something)" , "Having unfavourable reaction to foods or plants" .)