“How do you install Clojure?” Short answer: you don’t.
As noted in the general overview, you must have Java installed before you can use Clojure. If you don’t already have it:
sudo apt-get install openjdk-8-jre
The Clojure implementation itself is just a single jar file. The Clojure projects you create (described in the next chapter) will have access to a copy of that jar file which the project management tool automatically fetches and keeps around for you.
So, although you won’t need to install Clojure, you will need to install the project management tool, and that’s described in the next chapter.