classlojure lets you easily create a classloader with an alternate classpath and evaluate clojure forms in it. This classloader can even use a different version of clojure than your primary classloader.
(use 'classlojure)
(def clojure-13 (classlojure "file:clojure-1.3.0-alpha2-SNAPSHOT.jar"))
(eval-in clojure-13 '*clojure-version*)
;; {:interim true, :major 1, :minor 3, :incremental 0, :qualifier "alpha2"}
(eval '*clojure-version*)
;; {:major 1, :minor 2, :incremental 0, :qualifier ""}
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