Overdue debt (6)
I believe the answer is:
arrear
'overdue debt' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arrear that I've seen before include "Amount unpaid or undone" , "Backlog as of work" , "not been done" , "Backward once" , "A rarer kind of backlog" .)