Outstanding victory secured by defensive blunder (5)
I believe the answer is:
owing
'outstanding' is the definition.
('owing' can be a synonym of 'outstanding')
'victory secured by defensive blunder' is the wordplay.
'victory' becomes 'win' (I've seen this before**).
'secured by' is an insertion indicator (I've seen this in other clues).
'defensive blunder' becomes 'og' (own goal, football blunder).
'win' put within 'og' is 'OWING'.
(Other definitions for owing that I've seen before include "Due to, like debt" , "Unsettled; on account" , "Outstanding as a debt" , "Unpaid" , "Still outstanding" .)