This is likely due to an issue with some of your clan members. They are likely glitched and the game is not actually seeing them as being in the clan. This seems to be a common issue that many people are having.
See #9 (how to gain XP) and #17 (regarding not gaining XP) - Clan Issues and Fixes
A couple notes. One, the game does not refresh the Clan XP Needed very often so I would highly recommend using the Mobile app and refreshing your clan page (swipe down to refresh).
Secondly, we may have a fix for getting your clan members in your clan. See #17's 11/14 update: Clan Issues and Fixes
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.