it is, but generally (that is, without extreme hacking) only clojure.* ones, e.g. clojure.string, set, test, etc Don't use a ns form, instead (require [clojure.string :as str]) before the defproject form

Note this comment https://github.com/technomancy/leiningen/blob/b257a5ac02af51e1b89332c667e57c9e01f98d7e/sample.project.clj#L12 , inside the defproject you can use clojure code if it's prefixed with ~

thanks