Five hundred sheets of paper on cherished aspiration (5)
I believe the answer is:
dream
'cherished aspiration' is the definition.
(I've seen this in another clue)
'five hundred sheets of paper on' is the wordplay.
'five hundred' becomes 'd' (Roman numeral).
'sheets of paper' becomes 'ream' (a ream is a large amount of paper).
'on' is a charade indicator (letters next to each other).
'd'+'ream'='DREAM'
(Other definitions for dream that I've seen before include "Reverie or nightmare" , "It's experienced during sleep" , "Something imaginary" , "Vision, fantasy" , "Somniate" .)