Stack of paper (4)
I believe the answer is:
ream
'stack of paper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ream that I've seen before include "Twenty quires of paper" , "480 sheets of paper" , "stock of paper" , "480 or 500 sheets" , "Widen out (bore)" .)