Cut the rhubarb into 5cm/2in pieces and lay them in a 23cm/9in ovenproof dish. Top with the orange pieces.
Place the orange juice in a saucepan over a medium heat. Add the sugar, anise, cinnamon and vanilla seeds. Continue to heat until it becomes syrupy. Strain, then pour over the rhubarb.
For the topping, place the butter, flour, sugar and cinnamon in a bowl and use your hands to rub them together until it looks like breadcrumbs.
Add the almonds and spoon the crumble on top of the fruit and place in the oven for 25-30 minutes until golden-brown.
Serve with clotted cream or custard.