About a thousand sheets of paper (4)
I believe the answer is:
ream
'sheets of paper' is the definition.
'ream' can be an answer for 'paper' (a ream is an amount of paper). I'm not certain of the 'sheets of' bit.
'about a thousand' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'a thousand' becomes 'm' (Roman numeral for a thousand).
'rea'+'m'='REAM'
Can you help me to learn more?
(Other definitions for ream that I've seen before include "516 sheets of paper" , "Many bits of paper" , "Many sheets - enlarge hole" , "Five hundred sheets of paper" , "20 quires of paper" .)