Swimmer getting back in practice (4)

I believe the answer is:

carp

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

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

(Other definitions for carp that I've seen before include "Raise petty objections about goldfish" , "Pond fish" , "Swimmer" , "Fish - make captious criticism" , "Complain like goldfish" .)

I've seen this clue in the Evening Standard.

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