Preheat the oven to 200°C/400°F/Gas mark 6.
Trim the rhubarb, cut into short lengths and put in a pan with the sugar. Stir over a low heat until the sugar has dissolved, then cover and simmer for 8–10 minutes until the rhubarb is soft. Spoon into a deep 1.5-litre/ 2 1⁄2pt ovenproof dish.
Rub the butter into the flour until the mixture resembles fine breadcrumbs, then stir in the sugar and biscuits.
Sprinkle the crumble over the stewed rhubarb, and bake for 15 minutes or until the topping is golden brown. Serve with cream or ice cream.