CMSMainExtension.php

<?php

class CMSMainExtension extends Extension {

	/**
	 * @var array
	 */
	private static $allowed_actions = array(
		'cloneAction'
	);

	public function cloneAction(array $data, Form $form) {
		$record = DataObject::get_by_id("SiteTree", $data['ID']);

		// Do clone stuff
		
		return $this->owner->getResponseNegotiator()->respond($this->owner->request);
	}

}

config.yml

CMSMain:
  extensions:
    - CMSMainExtension