Leader of opposing side is outstanding (5)
I believe the answer is:
owing
'outstanding' is the definition.
('owing' can be a synonym of 'outstanding')
'leader of opposing side' is the wordplay.
'leader of' indicates taking the first letters.
'side' becomes 'wing' (I've seen this in other clues).
The initial letter of 'opposing' is 'o'.
'o'+'wing'='OWING'
'is' acts as a link.
(Other definitions for owing that I've seen before include "Due to, like debt" , "Unpaid" , "(Of money) outstanding" , "Unsettled; on account" , "This debt has yet to be paid" .)