The most common and most likely answer to this crossword clue is the 9 letter word ARREARAGE
I've seen this crossword clue in Financial Times
A
R
R
E
A
R
A
G
E
Buddy explains!
The straight part
We know that "What’s outstanding" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’s outstanding = arrearage.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ARREARAGE mean?
ARREARAGE noun- The condition of being in arrears.
- An item that is in arrears, as periodic payments on a debt or for taxes.