Underworld river audibly jams (4)

I believe the answer is:

styx

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

I can't explain the remainder of the clue.

(Other definitions for styx that I've seen before include "Infernal runner" , "classical stream" , "'River of Hate' in Hades" , "Hellish current" , "Mythological river ferried by Charon" .)

I've seen this clue in the Sydney Morning Herald.

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