Hades' river of forgetfulness (5)
I believe the answer is:
lethe
'hades' river of forgetfulness' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lethe that I've seen before include "Oblivion; river of the underworld" , "Ethel's mythical river of forgetfulness" , "River whose waters caused the souls of the dead to forget completely their life on Earth" , "Mythological river in Hades" , "where elephants never drink?" .)