Set the ham cut-side down in a large slow cooker.
Whisk cider, honey, brown sugar, Dijon, vinegar, butter, orange zest, cinnamon, cloves, garlic powder, and pepper in a saucepan.
Simmer over medium heat, stirring, until the sugar dissolves and the butter melts, 3–4 minutes.
Pour about half the warm glaze over the ham. Cover the slow cooker.
Cook on Low until heated through and very tender, 4½–5½ hours, spooning juices over the ham a couple of times.
Transfer ham to a board and tent with foil to rest.
Pour slow-cooker juices into the saucepan with the remaining glaze and bring to a lively simmer.
Thicken if desired: whisk cornstarch with cold water, then stir into the simmering glaze and cook until glossy, 2–3 minutes.
Brush the ham generously with glaze, then slice along the spiral cuts and serve with extra sauce.
Optional: For caramelized edges, broil the glazed ham on a sheet pan 2–3 minutes, watching closely.