Sheets covering large area (5)
I believe the answer is:
realm
'area' is the definition.
('realm' can be a synonym of 'area')
'sheets covering large' is the wordplay.
'sheets' becomes 'ream' (I've seen this in other clues).
'covering' means one lot of letters goes inside another.
'large' becomes 'l'.
'ream' placed around 'l' is 'REALM'.
(Other definitions for realm that I've seen before include "monarch's responsibility" , "A Nation" , "department" , "Dominion" , "where ruler holds sway?" .)