Create a new Leiningen project in Emacs and open its project.clj file

new-lein-project.el

(defun new-lein-project (name &optional template)
  "Interactively ask for a project and optional template name,
  then create a Leiningen project with the parameters
  and open its project.clj file."
  (interactive "sProject name: \nMTemplate (optional): ")
  (shell-command-to-string
    (concat "cd ~/src && " ;; change this to the directory where you keep your projects
            "lein new "
            template
            " " name))
  (find-file (concat "~/src/" name "/project.clj"))) ;; change it here, too

Caveat: I do not "know" elisp. There are probably at least 10 things here that can be done better.