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