One caught in the Elbe? (3)

I believe the answer is:

eel

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'caught in the elbe?' is the wordplay.
'caught in' indicates the answer is hidden within 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 "Moray or electric, for instance" , "Slippery swimmer (may be jellied)" , "Long smooth fish" , "Slippery character" , "Slithery fish" .)

I've seen this clue in the Evening Standard.

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