Infernal river (4)
I believe the answer is:
styx
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">infernal river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)