Place quartered nectarines in a large resealable bag.
In a small bowl, stir together amaretto, orange juice concentrate, and cloves. Pour over the nectarines and marinate in the refrigerator for 1 to 4 hours.
Meanwhile, stir together mascarpone, 2 tablespoons of marinade from the nectarines, and 1/4 cup of the slivered almonds. Refrigerate until ready to use.
Set up grill for direct grilling over medium heat and oil the grates.
Remove nectarines from marinade and place on hot grill. Cook for 3 to 4 minutes per side or until grill marks appear and nectarines have begun to soften.