TeX-add-style-hook.el

(eval-after-load
    "latex"
  '(TeX-add-style-hook
    "cleveref"
    (lambda ()
      (if (boundp 'reftex-ref-style-alist)
      (add-to-list
       'reftex-ref-style-alist
       '("Cleveref" "cleveref"
         (("\\cref" ?c) ("\\Cref" ?C) ("\\cpageref" ?d) ("\\Cpageref" ?D)))))
      (reftex-ref-style-activate "Cleveref")
      (TeX-add-symbols
       '("cref" TeX-arg-ref)
       '("Cref" TeX-arg-ref)
       '("cpageref" TeX-arg-ref)
       '("Cpageref" TeX-arg-ref)))))