If something crumbles, or if you crumble it, it breaks into a lot of small pieces.

If an old building or piece of land is crumbling, parts of it keep breaking off.

If something such as a system, relationship, or hope crumbles, it comes to an end.

If someone crumbles, they stop resisting or trying to win, or become unable to cope.

A crumble is a baked pudding made from fruit covered with a mixture of flour, butter, and sugar.