River of forgetfulness (5)
I believe the answer is:
lethe
'river of forgetfulness' is the definition.
(I've seen this in another clue)
This is the entire 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" .)