Exhibition spaces moving opening down to get reactions (9)

I believe the answer is:

allergies

I'm a little stuck... Click here to teach me more about this clue!

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

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

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 Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.