The important trick here is to make the crumble separately from the rhubarb - or apple - and then combine when you need to
220g unsalted butter
100g caster sugar
260g plain flour
Turn on oven to 180 deg C. Combine the butter and sugar in an electric mixer then add the flour until a crumble like texture is obtained. I suggest once it is well mixed grab handfuls of the mixture and press it together to help produce the lumps you want. Spread the mixture on a tray on baking paper and bake for around 10 minutes at 180 deg C.
Serve the Rhubarb or apple in dessert bowls, heated in the microwave. Once it is hot, sprinkle on the crumble mixture then serve with either cream or ice cream