Maintenance has been in arrears for months. Can it be claimed as a debt?

Yes. Act 303 s.69(1) provides that arrears of unsecured maintenance are recoverable as a debt from the defaulter; where they accrued before a receiving order was made against him, they are provable in his bankruptcy; and where they accrued before his death, they are a debt payable from his estate. S.69(2) also allows the legal personal representatives of the person entitled to recover them if the arrears accrued before that person's death.
The court also has power to order interim maintenance under s.70 while a maintenance application is pending. The copy of Act 303 we use is in English; the Malay here is our own translation, not an official text. Each state has its own Islamic family law enactment, so section numbers and provisions may differ from Act 303 (Federal Territories). The procedure of the Syariah Courts is a State matter under the Federal Constitution (Ninth Schedule, List II, item 1), so each state has its own civil procedure law. In the Federal Territories, the Syariah Court Civil Procedure (Federal Territories) Act 1998 [Act 585] s.148 lists an order of seizure and sale, hiwalah proceedings and, in certain cases, an order of committal; s.175–185 govern the judgment debtor summons. For forms and procedure in your state — ask your state Syariah Court (check with the state court).