Feature of underworld crook’s perhaps reaching our ears (4)

I believe the answer is:

styx

I'm a little stuck... Click here to teach me more about this clue!

'feature of underworld' is the definition.
'styx' can be an answer for 'underworld' (I've seen this before). I am not sure about the 'feature of' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for styx that I've seen before include "Underground river" , "Hell's river" , "The last river one crosses" , "Mythological river ferried by Charon" , "River separating Earth from Hades" .)

I've seen this clue in The Times.

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