Mythical river of forgetfulness? (5)
I believe the answer is:
lethe
'mythical 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 "Ethel's mythical river of forgetfulness" , "Classical river of forgetfulness" , "In Greek myth, the river of forgetfulness flowing through Hades" , "River whose waters caused forgetfulness" , "Underground flow of water" .)