Old part of building is outstanding (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'
'is' is the link.
(Other definitions for owing that I've seen before include "Unpaid" , "Outstanding as a debt" , "(Of money) outstanding" , "Due to, like debt" , "Undischarged" .)