What may be used by anglers to release river fish (4)

I believe the answer is:

eels

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

I don't understand how the rest of the clue works.

(Other definitions for eels that I've seen before include "They swim" , "Jellied --, traditional dish" , "Slithery fish" , "Slippery creatures" , "Fish (that may be jellied)" .)

I've seen this clue in The Times.

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