River down under? (4)
I believe the answer is:
styx
'river down under?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for styx that I've seen before include "River Charon rowed on" , "Underworld river ferried by Charon" , "One of the underworld rivers" , "Hellish current" , "One of the rivers of Hades" .)