A showplace for swimmers (8)
I believe the answer is:
aquarium
'a showplace' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for aquarium that I've seen before include "Tank for aquatic life" , "Large fish tank" , "Fish-viewing tank" , "A kind of fish zoo" , "Building containing fish tanks" .)