In order to allow user to interact with the xserver of your xorg instance, you must allow him to do so. You can do it in the insecure way by allowing all local hosts to access the xorg instance, or either allowing just a certain user to do so.
# allow all hosts in the local domain to access the xorg instance xhost +local: # allow only the specified localuser to get access to xorg display xhost +"si:localuser:USERNAME"
That's it! now you can allow the user “USERNAME” to access your display (or your clipboard), pretty much everything that is related to the desktop environment running