Adverse reaction from art venue moving opening much later (7)

I believe the answer is:

allergy

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'adverse reaction' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for allergy that I've seen before include "Producer of Hay Fever" , "Bad immune system response" , "Unfavourable reaction as to pollens or foods" , "unhealthy reaction" , "Abnormal reaction as to, say, pollen" .)

I've seen this clue in The Independent.

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