Outstanding -- nothing on flank (5)
I believe the answer is:
owing
'outstanding' is the definition.
('owing' can be a synonym of 'outstanding')
'nothing on flank' is the wordplay.
'nothing on' becomes 'o' (I've seen this in other clues).
'flank' becomes 'wing' (I've seen this before).
'o'+'wing'='OWING'
(Other definitions for owing that I've seen before include "Due to, like debt" , "This debt has yet to be paid" , "Outstanding as a debt" , "Unpaid" , "Undischarged" .)