Farewell note that could be under the futon (6)
I believe the answer is:
tatami
'farewell note that could be under the futon' is the definition.
The definition suggests a singular noun which matches the answer.
'farewell note' is the wordplay.
'farewell' becomes 'ta ta' (I have seen 'A Farewell' mean 'ta-ta' so perhaps 'farewell' could also mean 'ta-ta').
'note' becomes 'mi' (as in 'do re mi').
'tata'+'mi'='TATAMI'
Can you help me to learn more?
(Other definitions for tatami that I've seen before include "Floor-covering" , "Jap. room matting" , "Jap. floor mat" , "Japanese straw mat; a mat, it (anag.)" , "Japanese rush and straw matting" .)