Well basicly a dead clan is a clan that got inactive a lot, or doesn't have any members anymore. Seeing that no one will post in it anymore, and no active members are online ingame or anywhere, there is no point for it to exist anymore, so it's considered dead.
To revive it, it's leader should recruit (if he didn't quit that is), and try and make it more active. But if a clan is in the "dead clans" section, I dont think anyone can revive it seeing that no one, not even the leader, is in it anymore.