Duck in long river, dropping tail (3)

I believe the answer is:

nil

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

I don't understand the remainder of the clue.

(Other definitions for nil that I've seen before include "Zero, zilch" , "Nothing, no score" , "Non-existent" , "0" , "Nix, nada, nothing" .)

I've seen this clue in The Times.

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