Mythical river (7)
I believe the answer is:
acheron
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mythical river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for acheron that I've seen before include "A river of Hades" , "one classical route to underworld" .)