Overdue debt (6)
I believe the answer is:
arrear
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">overdue debt</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)