Mash cranberry sauce in a bowl; stir in cranberry juice, sugar, mustard, cloves, and salt.

Place pork roast into a slow cooker and pour cranberry mixture over top. Cook on Low for 6 to 8 hours. Transfer roast to a plate; cover and keep warm.

Skim fat from cranberry sauce in the slow cooker. Transfer sauce to a 2-cup liquid measure; add water if needed to reach the 2-cup mark. Pour into a saucepan and bring to a boil.

Whisk 2 tablespoons cold water and cornstarch into a paste; add to cranberry sauce. Cook and stir over medium-low heat until thickened, 5 to 10 minutes.

Slice pork and serve with cranberry sauce.