Gale Anne Hurd attended the BlogHer conference in Chicago. At the end of her keynote, she revealed a clip:

The clip opened with Daryl, Tyreese and others storming into an abandoned store for supplies. It is overrun with zombies and screams are heard.

The prison is also attacked by walkers. It seems someone is feeding them, but Rick and the gang don’t know who. Rick says they need more bodies to fight the walkers and children are showing being taught how to use guns.

It appears that the group decides to leave the prison because it is no longer safe. The clip teases it’s a whole new world full of fear and fighting.

I think it's a teaser trailer rather than a direct clip but there are a few tidbits that we didn't know.