1. Stir together honey and mustard; set aside all but 1/4 cup. Gently loosen chicken skin around breast. Rub 1/4 cup honey mustard evenly under and over skin, pushing it into legs and thighs. Sprinkle chicken inside and out with salt and pepper. Fold wings under and tie legs together.
2. Place chicken, breast side up, on a small rack or a ring made from foil inside slow cooker. Add 1/4 inch water. Cover; cook on high until an instant-read thermometer inserted into thigh registers 180ºF, 3 1/2 to 4 hours.
3. Preheat broiler to high. Transfer chicken, breast side up, to a foil-lined baking sheet. Combine 1 Tbsp. reserved honey mustard with olive oil. Brush mixture over chicken and broil to brown skin, 2 to 3 minutes. Let rest 10 minutes on a cutting board before serving. Drizzle remaining honey mustard over each portion of sliced meat.