Start playing notes (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'playing notes' is the wordplay.
'playing' indicates anagramming the letters.
'notes' with letters rearranged gives 'ONSET'.
(Other definitions for onset that I've seen before include "Appearance of" , "Beginning; violent attack" , "Beginning or early stages" , "First signs" , "17 down [ASSAULT]" .)