What do you gain from joining a clan.
I understand that first place gets prizes but do others get something?

Dont think of clans only for earning prizes. Think of it like a guild, as you would in any other game. You get to make friends and have a small community within the game. You can get direct help and chats from your clan, it's really just a social aspect. Every 300 battles ur clan wins u also get 5 extra battle credits so that's nice. But for every "kirito-at heart", being a solo player is just as fine. Depends what ur looking for in this game. Hope this helps!