Four notes gradually diminish (4)
I believe the answer is:
fade
'diminish' is the definition.
(I've seen this in another clue)
'four notes gradually' is the wordplay.
I cannot really understand how this works, but
'notes' could be 'fa' and 'fa' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for fade that I've seen before include "Gradually disappear from view" , "Diminish" , "Grow faint and disappear" , "flag" , "Lose colour or vitality" .)