Simons Framekiller is a small WordPress plugin that does nothing more than adding the frame killer code to the header of the page.
I created this plugin for a project but thought that perhaps others might want to use it too. The plugin is provided “as is” without warranty.
See: https://wordpress.org/plugins/simons-framekiller
What is a frame killer?
Wikipedia: A framekiller (or framebuster or framebreaker) is a technique used by websites and web applications to prevent their web pages from being displayed within a frame. A frame is a subdivision of a Web browser window and can act like a smaller window. A framekiller is usually used to prevent a website from being loaded from within a frameset without permission or as an attack, as with clickjacking.