Marking notes first is belittling (9)
I believe the answer is:
degrading
'belittling' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'marking notes first' is the wordplay.
I cannot really understand how this works, but
'marking' could be 'grading' ('grade' can be a synonym of 'mark') and 'grading' is located in the answer.
'notes' could be 'de' (two musical notes) and 'de' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for degrading that I've seen before include "Causing humiliation" , "Demeaning, humiliating" , "disgraceful" , "Debasing, demeaning" , "Small cucumber" .)