Vast area of 100 miles covered by Ordnance Survey twice (6)
I believe the answer is:
cosmos
'vast area' is the definition.
(the universe)
'100 miles covered by ordnance survey twice' is the wordplay.
'100' becomes 'c' (Roman numeral).
'miles' becomes 'm' (common abbreviation).
'covered by' indicates putting letters inside.
'ordnance survey' becomes 'OS' (abbreviation).
'twice' indicates the doubling of some letters.
'os' doubled is 'osos'.
'm' placed into 'osos' is 'osmos'.
'c'+'osmos'='COSMOS'
'of' acts as a link.
(Other definitions for cosmos that I've seen before include "The universe, whole of creation" , "Everything that exists" , "Universe as an ordered whole" , "The entire universe." , "well-organised world" .)