Imagine 500 sheets of paper (5)
I believe the answer is:
dream
'imagine' is the definition.
(dreaming is a kind of imagining)
'500 sheets of paper' is the wordplay.
'500' becomes 'd' (Roman numeral for 500).
'sheets of paper' becomes 'ream' (I am not sure about this - if you are sure you should believe this answer much more).
'd'+'ream'='DREAM'
Can you help me to learn more?
(Other definitions for dream that I've seen before include "Cherished desire" , "Aspire" , "Fantastic but vain hope" , "Reverie or nightmare" , "Nocturnal vision" .)