One swimming in the Elbe (3)

I believe the answer is:

eel

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

'in the elbe' is the wordplay.
'in' says the answer is hidden in the clue.
'EEL' can be found hidden inside 'the elbe'.

Can you help me to learn more?

(Other definitions for eel that I've seen before include "Long thin fish" , "Sea creature like conger" , "Proverbially slippery fish" , "Slippery type" , "Apode" .)

I've seen this clue in the Evening Standard.

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