River in Hades in Greek mythology that caused forgetfulness in those who drank from it (5)
I believe the answer is:
lethe
(Other definitions for lethe that I've seen before include "where elephants never drink?" , "In Greek myth, the river of forgetfulness flowing through Hades" , "River whose waters caused the souls of the dead to forget completely their life on Earth" , "Ethel's mythical river of forgetfulness" , "Oblivion; river of the underworld" .)