Book idiot opened, but not finished —notes follow this (4,4)
I believe the answer is:
bass clef
'not finished notes follow this' is the definition.
The definition suggests a singular noun which matches the answer.
'book idiot opened' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'idiot' becomes 'ass' (both can mean a stupid person).
'opened' becomes 'clef' (I can't explain this - if you can you should believe this answer much more).
'b'+'ass'+'clef'='BASS CLEF'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for bass clef that I've seen before include "Musical stave marking" , "Pitch symbol on stave (music)" , "shower of pitch" , "Musical notation with F on the fourth line of the stave" .)