Clans are one of the more fun aspects of the game; they allow you to have wars against other clans.
Clans may have up to 10 members, but they can have as few as just 1 member (the leader).
It costs 10 gold bars to create a clan, and you must be at least level 40. There is no level requirement to join a clan.
Wars are similar to bounties because once you are in a war, it is a fight to the death. Normally when you battle someone and they are under 25% of their health, you will not be able to attack them. In a war you can be attacked at all times, and even if you only have 1 health point left, you may be attacked and killed.
When a war is declared against another clan, it must be accepted within 2 hours or the declaration will expire.
The war will start one hour after the declaration of war is accepted. The war ends 24 hours after it has begun or until the war has been won.
Keep in mind that while you are in a war against another clan, winning attacks against lower level players will not be worth as much as a winning attack against a higher level player.
Handy tip for clan wars on mobile (this is hard to explain so bear with me):
On mobile devices, you can see the number of allies your opponent is bringing to battle from the battle page, but when you go to face an opponent from a clan wall you cannot.
To get around this, you can click on the opponents profile, go to "top rivals" and pick out a player who is not normally on the bounty list, go to that persons profile and view their top rivals. In this list of rivals you might be able to see the number of allies they will bring to battle, as long as your opponent is in their top rials list as well.