Old part of mansion is outstanding (5)
I believe the answer is:
owing
'outstanding' is the definition.
(an outstanding bill is still owing)
'old part of mansion' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'part of mansion' becomes 'wing' (a wing is one side of a large building).
'o'+'wing'='OWING'
'is' is the link.
(Other definitions for owing that I've seen before include "(Of money) outstanding" , "Unpaid" , "Outstanding as a debt" , "This debt has yet to be paid" , "Still outstanding" .)