Thanks for help.

experiment-struct.lisp

;;;; experiment-struct.lisp

;;; Replace structure slot accessor with generic function.
;;;
;;; Use it when only one or two slots need generic function otherwise
;;; use class.

(defpackage #:exp-struct
  (:use #:cl))

(in-package #:exp-struct)

(defmacro define-struct-generic (name (struct) &body body)
  (let ((ns (intern (concatenate 'string "%%" (symbol-name name))))
        (fn (symbol-function name))
        (sc (find-class struct nil)))
    ;; Ensure fn is function and struct is structure-class.
    (when (and (eql (type-of fn) 'function)
               (eql (type-of sc) 'structure-class))
      `(progn
         (defvar ,ns (copy-symbol ',name t))
         (fmakunbound ',name)
         (defgeneric ,name (,struct)
           (:method ((instance ,struct))
             (funcall ,ns instance))
           ,@body)))))

;;; Test with
;;; C-c C-c => slime-compile-defun

;;; No problem
(defstruct foo bar baz)

;;; No problem
(define-struct-generic foo-bar (foo)
  (:method ((instance null)) 0))