You hear someone at your work slander another co-worker. You take offence to this action. Is that harment?

Your boss should be sensitive to the fact that, while the target of the slander wasn't present, you were and it offended you. It shouldn't matter whether the attack was on you or not. Explain to your boss that it still bothered you, and that it would be the same as if they had told an off-color joke. You would have been just as offended by it. If he still does nothing, go over his head to the Human Resources department of your company.