Outstanding opener for Oxford United (4)
I believe the answer is:
owed
'outstanding' is the definition.
(as in an outstanding debt)
'opener for oxford united' is the wordplay.
'opener for' says to take the initial letters.
'united' becomes 'we'd' (united in marriage).
The initial letter of 'oxford' is 'o'.
'o'+'wed'='OWED'
(Other definitions for owed that I've seen before include "(Sum) due for payment" , "Was indebted" , "to be paid" , "Had an obligation to pay" , "Was obliged to pay" .)