Follow the steps below to fully remove yourself from any clan you are a part of and then try re-joining the clan you want.
See #17 in this thread for the full details:
11/14 UPDATE: If the app says someone is in your clan but they really aren't, here's how to fix it:
1 - Have the user Log into the mobile app
2 - Go to the My Clan section
3 - Click on the Leave Clan Icon
4 - Log out of the mobile app
5 - Repeat items 1-4 until you are no longer in a clan (they may find themselves in a few different clans)
6 - Have the person invite you into the clan you want to join.