It’s in the southeast, where building is going on (4)
I believe the answer is:
site
'where building is going on' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'it's in the southeast' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues**).
'in the' is an insertion indicator.
'southeast' becomes 'se' ('SE' can be a synonym of 'southeast').
'it' going within 'se' is 'SITE'.
Can you help me to learn more?
(Other definitions for site that I've seen before include "Locate - place" , "Spot, location" , "Location; locate" , "Whereabouts" , "Area set aside for building" .)