The first wrong note creates a discord (6)
I believe the answer is:
strife
'a discord' is the definition.
('strife' can be a synonym of 'discord')
'the first wrong note' is the wordplay.
'wrong' indicates anagramming the letters.
'note' becomes 'e' (musical note).
'first' with letters rearranged gives 'strif'.
'strif'+'e'='STRIFE'
'creates' is the link.
(Other definitions for strife that I've seen before include "Warfare" , "Trouble and conflict" , "Struggle" , "Lack of harmony" , "'Discord, conflict (6)'" .)