There is no social problem it is more incumbent upon us to understand than this of the role of c

There is no social problem it is more incumbent upon us to understand than this of the role of c

There is no social problem【that】 it is more incumbent upon us to understand than ｛this｝ of the role of custom.
这里的｛this｝指的是前面的 problem
social problem 和 problem of the role of custom （怎么说呢，里面的problem是同一个成分）相比较就知道：
social problem 翻译成社会的问题
problem of the role of custom 就应该翻译成为风俗作用的问题
句子意思是：没有任何一个社会问题比得上风俗作用的问题更有理由让我们去理解。