500 sheets of paper (4)
I believe the answer is:
ream
'500 sheets of paper' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ream that I've seen before include "20 quires of paper" , "516 sheets of paper" , "Many bits of paper" , "Widen (hole) with borer; twenty quires" , "Widen (hole); quantity of paper" .)