Notes wrong beginning (5)
I believe the answer is:
onset
'beginning' is the definition.
(I know that beginning can be written as onset)
'notes wrong' is the wordplay.
'wrong' indicates anagramming the letters.
'notes' anagrammed gives 'ONSET'.
(Other definitions for onset that I've seen before include "Beginning; violent attack" , "Beginning (of disease etc)" , "Beginning (of something often unpleasant)" , "Advent" , "17 down [ASSAULT]" .)