Crossword Genius

A long narrow slipper (3)

Ross

I believe the answer is:

eel

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

'slipper' is the definition.
(I've seen this before)

'a long narrow' is the wordplay.
I cannot really see how this works, but
'long' could be 'l' and 'l' is found within the answer.
The remaining letters 'ee' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for eel that I've seen before include "Elongated fish" , "Almost scaleless fish" , "Long thin fish" , "Slender fish" , "Unagi, in a sushi bar" .)

I've seen this clue in the Evening Standard.
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