DWIM find-function kind of affair.

find-definition-of-elisp-symbol.el

;; Interactively invoke tom/find-definition-of-elisp-symbol with point on an elisp symbol.

(defvar tom/find-definition-of-elisp-symbol-history '())

(defvar tom/find-definition-of-elisp-symbol-old-marker nil)

(defun tom/find-definition-of-elisp-symbol-add-marker ()
  (when tom/find-definition-of-elisp-symbol-old-marker
    (ring-insert find-tag-marker-ring tom/find-definition-of-elisp-symbol-old-marker)
    (setq tom/find-definition-of-elisp-symbol-old-marker nil)))

(defun tom/find-definition-of-elisp-symbol ()
  (interactive)
  (let* ((unksym (grep-tag-default))
         (funsym (function-called-at-point))
         (varsym (variable-at-point)))
    (add-hook 'find-function-after-hook 'tom/find-definition-of-elisp-symbol-add-marker)
    (setq tom/find-definition-of-elisp-symbol-old-marker (point-marker))
    (cond
     ((equal (symbol-name funsym) unksym)
      (find-function-do-it funsym nil 'switch-to-buffer))
     ((not (equal varsym 0))
      (find-function-do-it varsym 'defvar 'switch-to-buffer)))))