A bum's said to be something outstanding (7)
I believe the answer is:
arrears
'said to be something outstanding' is the definition.
I can't judge whether this definition defines the answer.
'a bum's' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'bum's' could be 'rears' ('rear' can be a synonym of 'bum') and 'rears' is present in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for arrears that I've seen before include "State of being behind with payments" , "Back payments due on a debt" , "Total unpaid debt" , "They're outstanding" , "In these, payment is overdue" .)