Outstanding to have good one inside (5)
I believe the answer is:
owing
'outstanding' is the definition.
(an outstanding bill is still owing)
'have good one inside' is the wordplay.
'have' becomes 'own' (to have something is to own it).
'good' becomes 'g' (abbreviation).
'one' becomes 'I' (Roman numeral).
'inside' is an insertion indicator.
'own'+'g'='owng'
'owng' enclosing 'i' is 'OWING'.
'to' acts as a link.
(Other definitions for owing that I've seen before include "This debt has yet to be paid" , "(Of money) outstanding" , "Outstanding as a debt" , "Unpaid" , "Due to, like debt" .)