verb

break or fall apart into small fragments, especially as part of a process of deterioration.

the plaster started to crumble

disintegrate,fall down,fall to pieces,fall apart,collapse,break down/up,tumble down,fragment,decay,fall into decay,deteriorate,degenerate,go to rack and ruin,decompose,rot,rot away,come apart at the seams,moulder,perish,come to dust

noun

a mixture of flour and fat that is rubbed to the texture of breadcrumbs and cooked as a topping for fruit.

sprinkle the crumble over the rhubarb