1. Peel and core apples. Cut into 1-inch chunks.
2. Trim and discard coarse ends and any bruised spots from rhubarb. Cut rhubarb into 1-inch pieces.
3. Grate 1 tablespoon peel from orange. Cut orange in half and ream 1/4 cup juice.
4. In a shallow 3-quart (9- by 13-in.) casserole, combine apples, rhubarb, orange peel, orange juice, brown sugar, cinnamon, cloves, ginger, and 2 tablespoons flour. Mix well.
5. In a food processor, combine 3/4 cup flour, pecans, and granulated sugar; whirl until nuts are finely ground. Or mince nuts with a knife and mix with flour and sugar in bowl. Add butter and whirl or rub mixture with your fingers until it forms fine crumbs. Squeeze crumbs into lumps and scatter over apple-rhubarb mixture.
6. Bake in a 375° oven until topping is well browned and juice bubbles at edges, about 40 minutes (35 minutes in a convection oven).
7. Meanwhile, in a chilled bowl with a mixer on high speed, whip cream until it holds soft peaks; add powdered sugar and vanilla. Serve, or cover and chill up to 4 hours; whisk before serving.
8. Let crisp cool 10 minutes. Serve warm or at room temperature. Spoon into bowls; top with whipped cream.