Hypersensitive reaction (7)
I believe the answer is:
allergy
'hypersensitive reaction' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for allergy that I've seen before include "Bad immune system response" , "Abnormal sensitivity to a food" , "Hypersensitive reaction of the body to a foreign substance" , "Hypersensitivity to a substance" , "Unfavourable reaction to, say, pollen" .)