Grumpy note, last of many (9)
I believe the answer is:
crotchety
'grumpy' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I don't see?
'note last of many' is the wordplay.
'note' becomes 'crotchet' (musical note).
'last of many' becomes 'y' (I am not sure about this - if you are sure you should believe this answer much more).
'crotchet'+'y'='CROTCHETY'
Can you help me to learn more?
(Other definitions for crotchety that I've seen before include "Cross" , "Bad-humoured or peevish" , "Irritable" , "Difficult" , "irascible" .)