Infernal river (4)
I believe the answer is:
styx
'infernal 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 "Infernal runner" , "River separating Earth from Hades" , "classical stream" , "River surrounding Hades" , "'River of Hate' in Hades" .)