My superficial understanding is that \'swank-clojure\' makes \'M-x slime-connect\' possible. I mean, it gives a connection to a clojure server something like \'lein swank\'. Is
swank-clojure.el is deprecated. Don't use it.
You need slime.el and you need to have swank-clojure "1.2.1" in your dev-dependcies in your project.clj file.
Swank is basically a server that you use slime to connect to from emacs. It it passed, from emacs, what you want run by the Lisp process that it's running.
You should use M-x slime-connect to connect to a swank server after starting with with lein swank.