Beginning to alter notes (5)
I believe the answer is:
onset
'beginning' is the definition.
(I know that beginning can be written as onset)
'alter notes' is the wordplay.
'alter' is an anagram indicator.
'notes' is an anagram of 'ONSET'.
'to' acts as a link.
(Other definitions for onset that I've seen before include "Beginning (of disease etc)" , "origin" , "Beginning or early stages" , "Cast stone in attack" , "starting?" .)