网友理解有误。这不是同位语从句，而是定语从句：

that we expect a definition of torture to answer，修饰one question。关系代词that 作answer 的宾语。

One question that we expect a definition of torture to answer is how to draw the line between forms of coercive interrogation that are legitimate under extreme circumstances and torture. 

我们希望酷刑定义能够回答的一个问题，是如何在极端情况下合法强制审讯和酷刑之间划清界限。