Addressing emperor with notes in order (4,3)
I believe the answer is:
tone row
'notes in order' is the definition.
The definition suggests a singular noun which matches the answer.
'addressing emperor with' is the wordplay.
'addressing' becomes 'to' (I can't explain this - if you can you should believe this answer much more).
'emperor' becomes 'nero' (Roman emperor).
'with' becomes 'w' (abbreviation).
'to'+'nero'+'w'='TONE ROW'
Can you help me to learn more?
(Other definitions for tone row that I've seen before include "Basis of serial music" , "Theme used in serial music" , "Sequence of twelve notes used in serial music" , "note sequence" , "Series of notes" .)