Fish returned: not completely smooth (4)

I believe the answer is:

eels

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

I don't understand the remainder of the clue.

(Other definitions for eels that I've seen before include "Sea creatures found in lees" , "They swim" , "Long fishes" , "Snakes found in lees" , "Congers" .)

I've seen this clue in The Hindu.

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