Adverse reaction (7)
'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" .)
'adverse reaction' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for backlash that I've seen before include "Violent reaction to event, like reverse whip" , "Repercussion" , "Violent reaction to an event - using whip?" , "negative comments?" , "Hostile reaction" .)