River of Hades
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">river of hades</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 "Mythological river in Hades" , "River separating Earth from Hades" , "River crossed at death" , "Hellish current" , "Mythological river ferried by Charon" .)