A long narrow slipper (3)
I believe the answer is:
eel
'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" .)