Run clang-format only when projects have a .clang-format in their root, while deferring the load of all packages with use-package

clang-format.el

(use-package cc-mode
  :defer t
  :init
  (add-hook 'before-save-hook #'clang-format-buffer-smart)
  :config
  (progn
    (defun clang-format-buffer-smart()
      "Reformat buffer only if .clang-format exists in the projectile root."      
      (when (file-exists-p (expand-file-name ".clang-format" (projectile-project-root)))
        (clang-format-buffer)))
    )
  :bind (:map c-mode-base-map
              ("C-c u" . clang-format-buffer))
  )

(use-package clang-format
  :ensure t
  :after cc-mode
  :config
  (setq clang-format-style-option "file")
  :commands clang-format clang-format-buffer clang-format-region)