Outstanding old part of building (5)
I believe the answer is:
owing
'outstanding' is the definition.
('owing' can be a synonym of 'outstanding')
'old part of building' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'part of building' becomes 'wing' ('wing' is part of 'building').
'o'+'wing'='OWING'
(Other definitions for owing that I've seen before include "Outstanding as a debt" , "Unpaid" , "Still outstanding" , "Due to, like debt" , "This debt has yet to be paid" .)