River of Hades (4)
I believe the answer is:
styx
'river of hades' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for styx that I've seen before include "Mythological river in Hades" , "River separating Earth from Hades" , "River crossed at death" , "Hellish current" , "Mythological river ferried by Charon" .)