While everything is clickable and you also want to support touch I would like to suggest 2 solutions:
Second click or double tap to deselect
Provide a dedicated icon somewhere to undo a selection.
Personally, I would go with 2nd solution as it's visible since the beginning to the user, and 1st one is alike 'discoverable' action (until they try, they won't ...
Drupal 8 has a core module built in to help with this call Tour which is built on the jQuery Joyride plugin.
If the website is built with bootstrap you might try the bootstraptour plugin git show your users all the features or guide them through some steps.
If your sites is not using a framework which offers a tour api you can build one with Joyride ...