A free-living swimmer? (3)

I believe the answer is:

eel

I'm a little stuck... Click here to teach me more about this clue!

'swimmer?' is the definition.
(eels swim)

'a free-living' is the wordplay.
'EEL' is hidden within the letters.
But, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for eel that I've seen before include "Snakelike fish" , "Snaky fish" , "A sea snake" , "Slippery type" , "Elongated fish" .)

I've seen this clue in the Evening Standard.

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