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