Imagine note on some paper (5)
I believe the answer is:
dream
'imagine' is the definition.
(dreaming is a kind of imagining)
'note on some paper' is the wordplay.
'note' becomes 'D' (musical note).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'some paper' becomes 'ream' (a ream is an amount of paper).
'd'+'ream'='DREAM'
(Other definitions for dream that I've seen before include "Aspire" , "Cherished hope; reverie" , "Nocturnal vision" , "Think vaguely" , "Cherished desire" .)