With the recent change in Clojure to use dedicated artifacts for Spec in an alpha sub-namespace, it is no longer easy to start a simple REPL just with the regular jar (e.g. java -jar $CLOJURE_JAR clojure.main). So how to start it now?
As of before you are better off to do a lein new somewhere, change the org.clojure/clojure dependency to the version you like and do a lein deps :tree to fetch all that is needed and display what things org.clojure/core now depends on. E.g.
[org.clojure/clojure "1.9.0-alpha17"] [org.clojure/core.specs.alpha "0.1.10" :exclusions [[org.clojure/clojure] [org.clojure/spec.alpha]]] [org.clojure/spec.alpha "0.1.123" :exclusions [[org.clojure/clojure]]]
All your artifacts end up in ~/.m2/repository. So next you have to build up your classpath to call the REPL. You can either run a lein with-profile uberjar cp in that dummy project to get the class path Leiningen is using (which often can be quite long depending on all the plugins you run. Or you can guess the locations from the versions above to build up the params for -cp for yourself.
M2="$HOME/.m2/repository" java -cp $M2/org/clojure/clojure/1.9.0-alpha17/clojure-1.9.0-alpha17.jar:$M2/org/clojure/spec.alpha/0.1.123/spec.alpha-0.1.123.jar:$M2/org/clojure/core.specs.alpha/0.1.10/core.specs.alpha-0.1.10.jar:. clojure.main
This is a very basic setup to get a basic REPL going, but paired with rlwrap this is a few times faster to fire up than a lein repl, which makes it perfect to quickly fool around with some ideas.
On the horizon to streamline this, there is https://github.com/clojure/tools.deps.alpha.