Howdy Simon,
I would love to help you with this.
The first step would be add the field to the page itself. You can do this by following our Themer’s Guide . Specifically you would want to override /the-events-calendar-community-events/src/views/community/edit-event.php With an override in place for that file you can add a new field.
I would give the new field the name/id of “excerpt”. If memory serves you will not even need to process the data submitted by this field, only add it to the front end. This is because we already have builtin the ability to process the fields the backend supports, including tags and I believe excerpt as well. So once you add the field and name it properly you will be done!
Does that all make sense? Will that work for you? Please let me know.
Cheers!
– Brook