Charon's river (4)
I believe the answer is:
styx
'charon's river' 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 "Hellish current" , "Mythological river ferried by Charon" , "Underworld river ferried by Charon" , "Hell's river" , "River separating Earth from Hades" .)