Imagine 500 sheets of paper (500) (5)
I believe the answer is:
dream
'imagine' is the definition.
(dreaming is a kind of imagining)
'500 sheets of paper 500' is the wordplay.
'500' becomes 'd' (Roman numeral for 500).
'sheets of paper 500' becomes 'ream' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'd'+'ream'='DREAM'
Can you help me to learn more?
(Other definitions for dream that I've seen before include "Fantasy; imagine" , "Aspiration" , "Bedroom vision?" , "Part of sleep" , "Vision - very pleasant person" .)