A hundred agree about area of land (7)
I believe the answer is:
acreage
'area of land' is the definition.
(land measured in acres)
'a hundred agree about' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'about' is an anagram indicator.
'agree' is an anagram of 'reage'.
'a'+'c'+'reage'='ACREAGE'
(Other definitions for acreage that I've seen before include "Number of area measurements" , "Farm" , "Extent of ground" , "Land area" , "Amount of land" .)