Cut clementines in half crosswise and juice them, being careful not to tear the peels; use juice to make clementine sorbet, or simply save for another use. With a small spoon, gently scrape out and discard remaining membranes. Place clementine cups (you will have 16) on a tray and freeze at least 20 minutes and up to overnight; cover with plastic wrap if freezing longer than 1 hour.
Make clementine sorbet, if using, or let purchased sorbet stand at room temperature 5 to 10 minutes to soften. Fill each citrus cup halfway with sorbet. Return cups to the freezer for at least 30 minutes.
Let ice cream stand at room temperature 5 to 10 minutes to soften. Spoon over sorbet to completely fill each citrus cup. Return cups to the freezer for at least 30 minutes.
Make meringue topping: In a large, clean, dry mixing bowl, with a mixer on high speed, whip egg whites until frothy, then slowly pour in sugar and salt. Continue whipping until whites are stiff and shiny. Pipe or spoon meringue on top of each citrus cup, being careful to cover all of the ice cream. The cups can be prepared to this point up to a day before serving and kept in the freezer (cover with plastic wrap if freezing longer than 1 hour).
When ready to serve, preheat oven to 350°. Place cups on their chilled tray in the hot oven and briefly bake until meringue is golden brown, about 3 minutes. Remove from oven and transfer two cups to each of 8 small dessert plates. Serve immediately.