Notes deserve a black mark (7)
I believe the answer is:
demerit
'a black mark' is the definition.
(a black mark marks a demerit or fault)
'notes deserve' is the wordplay.
'notes' becomes 'de' (two musical notes).
'deserve' becomes 'merit' (synonyms).
'de'+'merit'='DEMERIT'
(Other definitions for demerit that I've seen before include "Conduct that deserves censure" , "Something deserving censure" , "Black mark" , "Mark given for a fault" , "Fault, defect" .)