Question #1: What are clans for?
Clans are usually for people who have something common between them to gather together.

Question #2: Why should I join a clan? I mean, what benefits do I get from it?
Well, if u talk about naruto-arena, since there is no clan feature YET, there are no benefits in game.
However, you can make friends with your clan members and discuss about naruto-arena and naruto with them.

Question #3: Can I join more than 1 clan?
Well, i wouldnt do that if i were you. If u were the clan leader, would u like a clan member who is also in another clan?

Question #4: Can I leave the clan when I want? Or must I stay in the clan forever?
If i were you, i'd choose a clan carefully before joining instead of regretting later and quitting. Of course im sure u can quit but your ex clan members may be angry with you for that.

Question #5: How do you make a clan?
The clan feature hasnt been enabled on naruto-arena yet.