One letter clan tags got lame after [a] and [m]. You could use my old clantag from another game [AoS] or [AOS] or whatever you want. Ace Of Spades. If you're into ranks and stuff you can rank them by face cards, i.e joker, jack, queen, king, ace etc
If not, go with a simple word that is just awesome, you know, like [Magma] or something.