The bilingual river of Hades (5)
I believe the answer is:
lethe
'river of hades' is the definition.
(I've seen this in another clue)
'the bilingual' is the wordplay.
I cannot really understand how this works, but
an anagram of 'the' is 'eth' which is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lethe that I've seen before include "What could make one forget" , "River whose waters caused forgetfulness" , "Oblivion; river of the underworld" , "Ethel's mythical river of forgetfulness" , "Classical river of forgetfulness" .)