Arrears are payments you have fallen behind on. Rent arrears, fall into arrears, clear the arrears.
The word is plural only, and it means the overdue amount rather than the debt as a whole. Paid in arrears is a different and neutral sense, meaning paid after the work rather than before it, which is how most salaries run.
1nounarrearsPayments you have fallen behind on.
“They fell three months into arrears on their rent and received a warning letter.”
arrears is a debt.