Are you the Leader of the clan?
If you are the clan leader you cannot leave the clan, UNLESS you make someone else leader first.
You can pass on leadership to someone else in the clan, or if you are the only one in your clan, you can create a dummy account and pass on the leadership that way. Those are your only two options since there is no way to delete a clan.
Clan management is simply not working for anyone right now as it proves difficult to provide a bug-free website for Activision. We're also still waiting on feedback from them but since the support site is also full of bugs...