Outstanding victory defensive blunder secures (5)
I believe the answer is:
owing
'outstanding' is the definition.
('owing' can be a synonym of 'outstanding')
'victory defensive blunder secures' is the wordplay.
'victory' becomes 'win' (I've seen this before**).
'defensive blunder' becomes 'og' (own goal, football blunder).
'secures' is an insertion indicator.
'win' going into 'og' is 'OWING'.
(Other definitions for owing that I've seen before include "(Of money) outstanding" , "Unpaid" , "Undischarged" , "Due to, like debt" , "Outstanding as a debt" .)