What's a level 12 clan? And what should one do when badly losing a Clan War?
Yeah, by "Level 12 Clan" It means Clan Perk 12 which isn't the maximum (max level 15). Perks don't give extra powers in battle, they only make stuff like donation and war loot better. When a Clan is Level 12 it means they are worked a lot and they are probably a lot stronger then your clan and you should probably save your troops on the clan war and just have fun.
If you want to learn more about clan perks visit this page