Amount of land? A hundred agree to differ (7)
I believe the answer is:
acreage
'amount of land?' is the definition.
(I've seen this before)
'a hundred agree to differ' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'to differ' is an anagram indicator.
'agree' with letters rearranged gives 'reage'.
'a'+'c'+'reage'='ACREAGE'
(Other definitions for acreage that I've seen before include "expanse" , "Measure used by farmer" , "Land area" , "good quantity of land" , "Area measure" .)