Building work confined to flat area (6)
I believe the answer is:
pagoda
'building' is the definition.
(pagoda is a kind of building)
'work confined to flat area' is the wordplay.
'work' becomes 'go' ('go' can be a synonym of 'work').
'confined to' means one lot of letters goes inside another.
'flat' becomes 'pad' (person's home).
'area' becomes 'a' (maths abbreviation).
'go' placed into 'pad' is 'pagod'.
'pagod'+'a'='PAGODA'
(Other definitions for pagoda that I've seen before include "Hindu temple shaped like pyramid" , "Pyramidal temple with an upward curving roof" , "Sacred many-tiered temple" , "Oriental temple, often a tower" , "place for devotees?" .)