A swimming exhibition? (8)

I believe the answer is:

aquarium

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'a swimming exhibition?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

This is the entire clue.

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(Other definitions for aquarium that I've seen before include "Large fish tank" , "Tank for aquatic life" , "Fish tank or maring museum" , "A kind of fish zoo" , "Glass tank for fish" .)

I've seen this clue in the Evening Standard.

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