This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
A creditor was someone owed money by another.
As of 2371, D'Ghor was the principle creditor on gambling debts still remaining after Kozak's death, with evidence that it was part of a five year plot to bring down his house. (DS9: "The House of Quark")
In 2373, Hagath transferred Quark's earnings on a deal they had made directly to his creditors, who appreciated the move. He planned to continue doing so until Quark was debt-free, in an effort to remove all potential distractions. (DS9: "Business as Usual")