Surveyor's calculation (4)
I believe the answer is:
area
'surveyor's calculation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for area that I've seen before include "Part of a building, city etc" , "Expanse of a surface" , "field of study" , "Expanse or neighbourhood" , "Region, zone" .)