Add the cider, cloves, cinnamon, allspice and peppercorns to a medium pot. Bring to a boil, then lower the heat and simmer until reduced by a third, about 10 minutes.
Strain the cider through a sieve into a clean pot, then discard the spices. Add the red wine, port, half the clementines and half the apples. Cover and cook over medium-high heat until warmed through, 5 to 7 minutes.
Reduce the heat to low to keep warm. Garnish cups of warm mulled wine with the remaining fresh apple and clementine slices. (Do not serve the steeped fruit from the pot.)
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