simpler-indent.el

(defun my-indent-line ()
  "Make single-semicolon lisp comments indent like ;;-comments.
Mostly cribbed from `lisp-indent-line'."
  (interactive)
  (let ((indent (calculate-lisp-indent)) shift-amt end
        (beg (progn (beginning-of-line) (point))))
    (skip-chars-forward " \t")
    (if (or (looking-at "\\s<\\s<") (not (looking-at "\\s<")))
        (lisp-indent-line)
      (if (listp indent) (setq indent (car indent)))
      (setq shift-amt (- indent (current-column)))
      (if (zerop shift-amt)
          nil
        (delete-region beg (point))
        (indent-to indent)))))
(add-hook 'clojure-mode-hook
          (lambda ()
            (set (make-local-variable 'indent-line-function)
                 'my-indent-line)))