Outstanding old group (5)
I believe the answer is:
owing
'outstanding' is the definition.
(an outstanding bill is still owing)
'old group' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'group' becomes 'wing' (I can't justify this - if you can you should give a lot more credence to this answer).
'o'+'wing'='OWING'
Can you help me to learn more?
(Other definitions for owing that I've seen before include "Unpaid" , "Unsettled; on account" , "Undischarged" , "(Of money) outstanding" , "This debt has yet to be paid" .)