Preheat the oven to 180C/355F/Gas 4.
Cut the rhubarb into 5cm/2in pieces and lay in a 22cm/9in square ovenproof dish.
Meanwhile place the orange juice, sugar, star anise, cinnamon and vanilla pod into a saucepan over a medium heat and bring to the boil. Reduce the heat and simmer until the liquid thickens to a syrup, then pour the syrup over the rhubarb.
For the topping, place the flour, sugar, almonds and cinnamon into a bowl and mix well. Add the butter and rub it into the dry ingredients until the mixture resembles breadcrumbs.
Sprinkle the topping over the rhubarb and transfer to the oven to bake for 35-40 minutes, or until the topping is golden-brown.
To serve, spoon out generous portions of the rhubarb crumble into bowls and serve with custard.