Preheat the griller. Line a small baking tray with aluminium foil (the caramelised drippings can be hard to clean up). Cut grapefruit in half crosswise. Use a paring knife or grapefruit knife to separate the flesh from the rind and membrane. Drizzle 1 teaspoon honey over each grapefruit half and spread evenly. Sprinkle with cinnamon. Set grapefruit halves on the baking tray and grill until grapefruit skin is lightly browned in places and the grapefruit is warmed through, 5 to 7 minutes.