Outstanding old part of large house? (5)
I believe the answer is:
owing
'outstanding' is the definition.
(an outstanding bill is still owing)
'old part of large house?' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'part of large house?' becomes 'wing' (a large house might have separate wings).
'o'+'wing'='OWING'
(Other definitions for owing that I've seen before include "Unsettled; on account" , "Due to, like debt" , "This debt has yet to be paid" , "Unpaid" , "(Of money) outstanding" .)