This is a very basic crumble -- and it doesn't need improving.
For the crumble: 100 g plain flour; 60 g butter; 1 tbsp caster sugar. I like a buttery crumble. As when making pastry, I cut the butter into little cubes and put them back into the fridge to chill for half an hour. Then I rub them into the flour. It doesn't matter if some blobs of butter remain.
You can also use a food processor. But I quite enjoy rubbing butter into flour; and I don't enjoy getting the processor out of the cupboard, assembling it, disassembling it, and washing it.
Stir in the sugar.
For the apple: 3 Bramley apples. Bramleys are best, I think. I have used any kind of eating apple that has been in the house. But they disgorge their moisture, and become dry, in my experience; whereas Bramleys retain a moist texture. Also, no other apple has that mouth-tickling tartness.
Peel them, halve them, quarter them, core them, cut them into segments, and throw them into acidulated water to stop them discolouring. (I use the juice of half a lemon or, if I don't have that, some vinegar.) Transfer them to a pan, with some of the water still clinging to them, cover, and put on a medium heat. After a couple of minutes, sprinkle over a tbsp of caster sugar (which might have caught on the dry pan). Stir in a clove or two and a dash of cinnamon if you like.
Cook the apples until they start to soften, stopping while they retain their shape. Taste for sweetness. You should not need more liquid. If you do, stir in a little orange juice. If there is too much liquid, pour it off into another pan, boil it to reduce it to a syrup, and recombine it with the apple.
Tip the apples into an oven dish. Sprinkle on the crumble, dot it with a few shavings of butter, and bake at gas mark 6/200 C for about 30 minutes, or until the top is brown.