Simply set the trigger property to be serial. The messages will be processed in the same order, one at a time, as they were received by Broker–assuming that the messages are published by a single publisher.
When the partner is down, suspend the trigger.
IS cluster is immaterial, more or less.
Please review the documentation.
#webMethods#Integration-Server-and-ESB#Broker-JMS#Universal-Messaging-Broker