In which the ancients drowned their memories? (5)
I believe the answer is:
lethe
'in which the ancients drowned their memories?' is the definition.
The definition suggests a singular noun which matches the answer.
'in which the' is the wordplay.
'in which' becomes 'le' (I can't justify this - if you can you should give a lot more credence to this answer).
'le'+'the'='LETHE'
Can you help me to learn more?
(Other definitions for lethe that I've seen before include "Ethel's mythical river of forgetfulness" , "Underground flow of water" , "What could make one forget" , "River whose waters caused forgetfulness" , "where elephants never drink?" .)