Scrap paper behind folio (4)
I believe the answer is:
bumf
'scrap paper' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'behind folio' is the wordplay.
'behind' becomes 'bum' ('bum' can be a synonym of 'behind').
'folio' becomes 'f' (abbreviation listed in Chambers).
'bum'+'f'='BUMF'
Can you help me to learn more?
(Other definitions for bumf that I've seen before include "Official documents" , "Printed material" , "Too much information!" , "Boring documents etc" , "Junk-mail" .)