note: both use expand-region to grab the clojure that the cursor is on or immediately after, and grep for whatever was selected.

grep recursively starting at the directory where the project.clj file lives

(defun grep-in-project ()
  (interactive)
  (er/mark-clj-word)
  (let* ((project-root (locate-dominating-file
                         (file-name-directory (buffer-file-name)) "project.clj")))
    (if project-root
        (grep (concat "grep -nH -e "
                      (buffer-substring-no-properties (region-beginning) (region-end))
                      " -R " project-root))
      (message (concat "no project.clj found at or below " (buffer-file-name))))))

(global-set-key (kbd "C-c g") 'grep-in-project)

grep recursively, but allow the user to select the root directory (defaulting to the location of the project.clj file). I often use this one for selecting only the src or test directories of my project.

(defun grep-in (project-root)
  (interactive (list (read-directory-name "Project Root: "
                                          (locate-dominating-file
                                            default-directory
                                            "project.clj"))))
  (er/mark-clj-word)
  (grep (concat "grep -nH -e "
                (buffer-substring-no-properties (region-beginning) (region-end))
                " -R " project-root)))

(global-set-key (kbd "C-c C-g") 'grep-in)