The reason my apple pie is famous—cinnamon.
这里的my apple pie 是the reason的同位语么?
The reason my apple pie is famous—cinnamon.
=The reason (why /that) my apple pie is famous is that it is made of cinnamon.
【 (why /that) my apple pie is famous 是定语从句,修饰 The reason】
【The reason (why /that)的用法详见《牛津英语用法指南》第四版567节】