PreparationPreheat oven to 375°F. Boil Port, 1/2 cup brown sugar, and allspice in large skillet until reduced to 2/3 cup, about 10 minutes. Place plums in large bowl. Sprinkle flour over; toss to coat. Drizzle 1/3 cup syrup over plums; toss to coat. Reserve remaining syrup.
Unroll cold pie crust onto parchment-lined baking sheet. Spoon plums into center of crust, leaving 1 1/2-inch border; drizzle any remaining syrup from bowl over plums. Fold crust edges over plums. Brush crust with water; sprinkle crust and plums with remaining 1 tablespoon brown sugar.
Bake tart until crust is golden and syrup is bubbling, about 45 minutes. Cut warm or room-temperature tart into wedges. Serve with scoop of vanilla ice cream, drizzling additional syrup over.