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 in other clues). I'm not certain 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" .)