A hundred agree about extent of land (7)
I believe the answer is:
acreage
'extent of land' is the definition.
(I've seen this in another clue)
'a hundred agree about' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'about' indicates an anagram.
'agree' with letters rearranged gives 'reage'.
'a'+'c'+'reage'='ACREAGE'
(Other definitions for acreage that I've seen before include "Area measure" , "Measure used by farmer" , "Farm" , "plot" , "Extent of ground" .)