This should be doable using the secondary usergroup...
You can use the "Public Joinable" ones and set the clan leaders as the group leaders, then they can approve, deny access to those who request access.
Then you can also create new calendars for each clan that only those in that secondary usergroup can view/post to.
In other words, registered members would have NO access to the calendar, but the secondary usergroup would. The permissions of thesecondary usergroup override the registered group so they would be able to see it.