Geometric calculation (4)
I believe the answer is:
area
'geometric calculation' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for area that I've seen before include "Extent of a two-dimensional surface" , "Space for a specific purpose, say picnic" , "Expanse of a surface" , "Geographical region" , "environs" .)