Swimmer making a bit of a stink (5)

I believe the answer is:

tench

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

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

(Other definitions for tench that I've seen before include "catch" , "Fish of the carp family" , "something fishy" , "Freshwater fish" , "One in school" .)

I've seen this clue in the Evening Standard.

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