Not the cockney gentleman with the fish (3)

I believe the answer is:

eel

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

'fish' is the definition.
(I know that eel is a type of fish)

I can't explain the rest of the clue.

(Other definitions for eel that I've seen before include "Ribbon-like fish" , "Electric swimmer, perhaps?" , "slipper" , "Long smooth fish" , "Snakelike fish" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app