Amount of paper (4)
I believe the answer is:
ream
'amount 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 "Widen (hole); quantity of paper" , "Five hundred sheets of paper" , "Quantity of paper, about 500 sheets" , "Lots of paper" , "480 sheets of paper" .)