Underworld river (4)
I believe the answer is:
styx
'underworld river' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for styx that I've seen before include "classical stream" , "One of the rivers of Hades" , "Underworld river; sounds like remote area" , "Charon's 26 carried dead souls across this river" , "River of hell" .)