Read 1,000, not 500 + 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)
'read 1000 not 500' is the wordplay.
'1000' becomes 'M' (synonyms).
'not' is a deletion indicator.
'500' becomes 'D' (Roman numeral for 500).
'read'+'m'='readm'
'readm' with 'd' removed is 'REAM'.
(Other definitions for ream that I've seen before include "Many bits of paper" , "516 sheets of paper" , "Lots of paper" , "Quantity of paper, about 500 sheets" , "Widen out (bore)" .)