Wallpaper hanger's calculation (4)
I believe the answer is:
area
'wallpaper hanger'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 "Geographical region" , "Part of a building, city etc" , "Expanse or neighbourhood" , "Surface measurement" , "District; plot" .)