Place pork roast in a slow cooker. Mash cranberry sauce in a bowl; stir in sugar, cranberry juice, mustard, cloves, and salt. Pour cranberry mixture over pork roast.
Cook on Low for 6 to 8 hours. Transfer roast to a plate; cover and keep warm.
Skim fat from cranberry mixture in slow cooker. Measure 2 cups of cranberry mixture, adding water if necessary. Pour into saucepan and bring to a boil. Whisk cold water with cornstarch until pasty; pour into cranberry mixture. Cook and stir until thickened, 5 to 10 minutes. Serve alongside pork.