Say what Pooh played with in mythical river (4)

I believe the answer is:

styx

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'mythical river' is the definition.
(I've seen this in another clue)

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

Can you help me to learn more?

(Other definitions for styx that I've seen before include "Underground river" , "One of the underworld rivers" , "Hell's river" , "'River of Hate' in Hades" , "Hellish current" .)

I've seen this clue in The Guardian.

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