Could it be fun to sleep on? (5)
I believe the answer is:
futon
'sleep on?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'could it be fun to' is the wordplay.
'could it be' indicates an anagram.
'fun'+'to'='funto'
'funto' anagrammed gives 'FUTON'.
Can you help me to learn more?
(Other definitions for futon that I've seen before include "A Japanese bed-mattress" , "Foldable mattress" , "Japanese sofa bed" , "Kind of Japanese bed convertible to couch" , "Kind of Japanese chair-bed" .)