Four notes get quieter (4)
I believe the answer is:
fade
'four' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'notes get quieter' is the wordplay.
'notes' becomes 'ad' (two musical notes).
'get' is an insertion indicator.
'quieter' becomes 'fe' (I can't explain this - if you can you should believe this answer much more).
'ad' inserted into 'fe' is 'FADE'.
Can you help me to learn more?
(Other definitions for fade that I've seen before include "Dwindle" , "Gradually disappear from view" , "Discolour" , "Grow dull" , "Lose colour gradually" .)