Combine & Simmer: In a small saucepan, combine all ingredients: maple syrup, mustard, apple cider vinegar, cinnamon, and cloves. Whisk well to combine.
Reduce the Glaze: Place the saucepan over medium heat. Bring the mixture to a gentle simmer, then reduce the heat to medium-low. Cook for 5-7 minutes, stirring occasionally, until the glaze has thickened slightly. It should coat the back of a spoon and will continue to thicken as it cools. Remove from heat. (Tip: The glaze can be made up to 3 days ahead. Store covered in the refrigerator and gently reheat before using.)
Prepare the Ham: Preheat your oven according to your ham's package directions (typically 325°F / 160°C). Place the ham, flat-side down, in a roasting pan. If your ham has a thick fat cap, use a sharp knife to score it in a 1-inch diamond pattern, being careful not to cut into the meat.
Apply the Glaze: Using a pastry brush or spoon, apply a generous layer of the warm glaze all over the surface of the ham.
Bake & Baste: Bake the ham as directed, typically about 15-18 minutes per pound for a pre-cooked ham. Baste the ham with more glaze every 20-30 minutes. For a final, beautiful finish, during the last 15-20 minutes of baking, increase the oven temperature to 425°F (220°C). Apply one last layer of glaze—this will create a perfectly sticky, caramelized crust.
Rest and Serve: Once baked, remove the ham from the oven, tent loosely with foil, and let it rest for 15-20 minutes before carving. Serve any remaining warmed glaze on the side.