River in Hades in Greek mythology that caused forgetfulness in those who drank its waters (5)
I believe the answer is:
lethe
(Other definitions for lethe that I've seen before include "In Greek myth, the river of forgetfulness flowing through Hades" , "Underground flow of water" , "Classical river of forgetfulness" , "where elephants never drink?" , "Ethel's mythical river of forgetfulness" .)