Outstanding opinion? (5)

I believe the answer is:

owing

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'outstanding' is the definition.
(an outstanding bill is still owing)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for owing that I've seen before include "Unsettled; on account" , "Due to, like debt" , "Outstanding as a debt" , "Undischarged" , "Unpaid" .)

I've seen this clue in the Sydney Morning Herald.

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