Badly laid out galleries — they bring adverse reactions (9)

I believe the answer is:

allergies

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'reactions' is the definition.
(I have seen 'Unhealthy reactions' mean 'allergies' so perhaps 'reactions' could also mean 'allergies')

'badly laid out galleries they bring adverse' is the wordplay.
'galleries' can be anagrammed to 'ALLERGIES'.
However, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Another definition for allergies that I've seen is " Unhealthy reactions".)

I've seen this clue in The Times.

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