George, Duke of Clarence (a.k.a. Clarence) is the brother of King Edward IV and Richard III. At the play's beginning, he's accused of treason because someone prophesied that King Edward's heirs would be killed by a family member whose name starts with the letter "G." The prophecy turns out to be true, but it's actually Richard, Duke of *G*loucester who orders the young princes' deaths, not *G*eorge, Duke of Clarence.
After Richard issues his death warrant, poor Clarence in stabbed and then drowned in a big vat of wine in Act 1, Scene 4. This is pretty ironic given that Clarence had a dream about drowning in the ocean prior to his murder. Check out "Symbolism, Imagery, Allegory" for all the gory details.