Unfortunately there are few to nothing libraries, natively suitable to be used in Clojure development that implement MQTT server side functionalities.
The Moquette library is Java MQTT broker based on an event model by Netty. The library has a good performance and is designed with embedding support available out of the box. Its configuration is compatible with well known Mosquitto Open Source MQTT server.
When being combined - these tools open the way to painless M2M communications for services written in Clojure.
As always we prefer to keep things as tiny and as simple as it ever possible, so the library interface is very ascetic and pure.