Note, it may be outstanding (4)
I believe the answer is:
bill
I believe this clue is a double definition.
'note' is the first definition.
(I know that note can be written as bill)
'it may be outstanding' is the second definition.
(I've seen this in another clue)
(Other definitions for bill that I've seen before include "Chap" , "Bird's beak or statement for payment" , "one settles with in America?" , "informing of damage" , "Account; beak" .)