Security is an important aspect to keep an eye for, and this time it’s about preventing clickjacking on Drupal and other Apache web applications.
Edit apache’s configuration file, which may be your declared vhost or such, usually at a location like
/etc/httpd/conf.d/default.conf and make sure the following
<IfModule mod_headers.c> Header always append X-Frame-Options SAMEORIGIN </IfModule>
This will disable embedding your website as an iFrame.