Long fish – what makes you see length? (4,3)

I believe the answer is:

sand eel

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

'long fish' is the definition.
'sand eel' can be an answer for 'fish' (I've seen this before). I'm not sure about the 'long' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sand eel that I've seen before include "Shallow-water fish" , "swimmer" , "Very small fish that burrows into beaches" .)

I've seen this clue in The Guardian.

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