Adverse reaction (7)
I believe the answer is:
allergy
'adverse reaction' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for allergy that I've seen before include "Abnormal body reaction" , "Largely to do with adverse reaction" , "Immune reaction" , "Cause of ill-feeling" , "Abnormal reaction as to, say, pollen" .)