500 paper sheets seen in vision (5)
I believe the answer is:
dream
'vision' is the definition.
(dream is a kind of vision)
'500 paper' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'paper' becomes 'ream' (I've seen this before).
'd'+'ream'='DREAM'
'sheets seen in' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dream that I've seen before include "Fantasise, imagine" , "Sleeping experience" , "Sleeper's vision" , "Vision, fantasy" , "Cherished hope; reverie" .)