A clod is a lump or chunk of something. You begin the process of making a bowl by throwing a clod of clay onto a potter's wheel.
Clod usually describes a mass or ball of dirt: "She intended to plant vegetables in her yard, but found the soil was full of hard clods and stones." An annoyingly awkward person can also be called a clod: "He always says the wrong thing — he's such a clod." The word comes from clot, which originally meant "a mass," and comes from the German Klotz, "lump or block."