The swimmer may finish up battered (4)

I believe the answer is:

fish

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'the swimmer' is the definition.
(fish is a kind of aquatic vertebrate)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for fish that I've seen before include "Eg, sturgeon" , "Burbot or turbot, for example" , "Aquatic creature(s)" , "To try to catch" , "Possibly the sole" .)

I've seen this clue in the Evening Standard.

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