River of forgetfulness (5)
I believe the answer is:
lethe
'river of forgetfulness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lethe that I've seen before include "Classical river of forgetfulness" , "What could make one forget" , "In Greek myth, the river of forgetfulness flowing through Hades" , "River whose waters caused forgetfulness" , "Mythological river in Hades" .)