River to Hades (4)
I believe the answer is:
styx
'river to 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 ferried by Charon" , "River of hell" , "One of the underworld rivers" , "'River of Hate' in Hades" , "Mythological river in Hades" .)