River of Hades (4)
I believe the answer is:
'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" .)