Hypersensitivity (7)
I believe the answer is:
allergy
'hypersensitivity' is the definition.
(allergy is a kind of hypersensitivity)
This is all the clue.
(Other definitions for allergy that I've seen before include "Abnormal body reaction" , "Pathological aversion" , "Unfavourable reaction to, say, pollen" , "Abnormal reaction as to, say, pollen" , "Unfavourable reaction as to pollens or foods" .)