;; Elisp

(defun slime-xref-undefine-method ()
  (interactive)
  (let (begin end)
    (beginning-of-line)
    (search-forward "(")
    (backward-char)
    (setf begin (point))
    (forward-sexp)
    (setf end (point))
    (slime-eval (list 'cl-user::slime-xref-undefine-method
                      (buffer-substring-no-properties begin end)))
    (beginning-of-buffer)
    (search-forward "DEFGENERIC ")
    (call-interactively 'slime-edit-definition)
    (goto-char begin)
    (beginning-of-line)))

;; Common lisp

(defun cl-user::slime-xref-undefine-method (xref-string)
  (destructuring-bind (defmethod gf-name &rest specializers)
      (read-from-string xref-string)
    (let* ((gf (fdefinition gf-name))
           (qualifiers
             (when (member (first specializers) '(:around :after :before))
               (list (pop specializers))))
           (method (find-method gf 
                                qualifiers
                                (mapcar #'find-class specializers))))
      (remove-method gf method)
      nil)))