Heat oven to 350F.
Combine brown sugar, oats, flour, melted shortening, and cinnamon. Remove about 1 cup of crumble for later. Press the remaining mixture into the bottom of a greased pie tin or cake pan to form the crust.
Dice rhubarb and spread over the crust.
Combine sugar, cornstarch, water, and vanilla in a saucepan. Over low heat, cook until thick and clear. Pour mixture over rhubarb.
Add remaining crumble mixture over the top.
Bake for 1 hour.