Rubbish teacher gets sent up, one who's not yet settled (6)

I believe the answer is:

debtor

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'rubbish teacher' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I cannot understand how the remainder of the clue works.

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(Other definitions for debtor that I've seen before include "One owing money" , "One of The Borrowers" , "One who owes money" , "Anyone owing money" , "Person owing money" .)

I've seen this clue in The Guardian.

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