The most common and most likely answer to this crossword clue is the 4 letter word EELS
I've seen this crossword clue in The Guardian
E
E
L
S
More clues leading to the result EELS
- Fish slumber mostly upside down!
- Slippery types, scoundrels in the East End
- Feel sick, having eaten fish
- Keep tabs on vacuous liars — they’re slippery
- Boats heading off to fish
- Slippers into which these go
- They may be shocking swimmers
- Fish where else — at sea?
- Fish experiences having head cut off
- After removing head, skins fish
Buddy explains!

The straight part
We know that "Fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fish = eels.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does EELS mean?
EELS noun- Any freshwater or marine fish of the order Anguilliformes, which are elongated and resemble snakes.
- The European eel, Anguilla anguilla.
- To fish for eels.
- To move with a sinuous motion like that of an eel.