“Don’t forget”, says the note, “to rebel against” (8)
I believe the answer is:
memorise
'don't forget says the note' is the definition.
I can't judge whether this definition defines the answer.
'to rebel against' is the wordplay.
'to' becomes 'memo' (I can't justify this - if you can you should believe this answer much more).
'rebel' becomes 'rise' (synonyms).
'against' is a charade indicator (letters next to each other).
'memo'+'rise'='MEMORISE'
Can you help me to learn more?
(Other definitions for memorise that I've seen before include "Commit information to the mind" , "Learn by heart" .)