MIX sugar and cream of tartar in medium saucepan. Gradually stir in water. Bring to boil on high heat; simmer on low heat 5 min. or until mixture is reduced to 1-1/2 cups. Stir in zest and juice; cool 30 min.
HEAT oven to 425°F. Roll out 1 crust on lightly floured surface to 11-inch circle; place in 9-inch pie plate. Place cracker crumbs in crust. Pour sugar syrup over crumbs; top with butter and cinnamon.
ROLL out remaining crust to 10-inch circle; place over pie. Seal and flute edge. Cut several slits in top crust to permit steam to escape.
BAKE 30 to 35 min. or until golden brown. Cool.
CHOCOLATE-WALNUT MOCK APPLE PIE
Prepare as directed, sprinkling 1/2 cup each chopped PLANTERS Walnuts and BAKERS Semi-Sweet Chocolate Chunks over cracker crumbs before covering with sugar syrup.