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 eventing model with Netty. The library has a good performance and is desegned with embedding support 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 have things tiny and as simple as possible.