A number of sheets - about a thousand? (4)
I believe the answer is:
ream
'a number of sheets' is the definition.
'ream' can be an answer for 'sheets' (I've seen this before). I'm unsure of the 'a number of' bit.
'about a thousand?' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'thousand?' becomes 'M' (Roman numeral for a thousand).
're'+'a'+'m'='REAM'
Can you help me to learn more?
(Other definitions for ream that I've seen before include "20 quires of paper" , "stock of paper" , "Twenty quires of paper" , "Five hundred sheets of paper" , "Widen (hole); quantity of paper" .)