Clojure generally doesn't use monads and has its own way of dealing with mutable state. Clojure doesn't prevent side-effects, but it does make them somewhat unidiomatic in most cases. In any case, Clojure makes it pretty easy to write pure-functional code and carefully manage the mutable state and side-effects that you have.

Having said that, I don't know anything about Arcadia and how the Clojure code interacts with Unity and how clean the paradigm separation is...