Crossword Genius

Swimmer, sleek, goes back out of bounds (3)

Ross

I believe the answer is:

eel

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

'swimmer sleek goes back out of' is the wordplay.
'out of' indicates a hidden word.
'goes back' says the letters should be written backwards.
'lee' is hidden in the letters of 'swimmer sleek'.
'lee' backwards is 'EEL'.

Can you help me to learn more?

(Other definitions for eel that I've seen before include "Apode" , "Electric swimmer, perhaps?" , "Slippery figure" , "Evasive type" , "Slimy fish" .)

I've seen this clue in The Daily Mail.
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