There was problem after problem to overcome, not least of which was that their store of Polyjuice Potion was greatly depleted.

主句语法结构分析：

引导词:There

系动词：was

主语：problem after problem

后置定语：to overcome

非限制性定于从句语法结构分析：

主语：not least of which

系动词：was

表语从句：that their store of Polyjuice Potion was greatly depleted

还原之后结构为：There was problem after problem to overcome and not least of them was that their store of Polyjuice Potion was greatly depleted.

参考译文：有一个又一个的难题/问题需要克服，最大的难题就是他们复方药水储备已经大大被耗尽了。

参考译文：有一个又一个需要克服的问题/难题，尤其困难的是他们复方药水储备已经大大被耗尽了。

这句话结构是什么？后半句加粗部分是什么从句？句中of起什么作用？

解答如下：

of which 为介词+关系代词引导的非限制性定语从句。后面为that引导表语从句用来说明not least of which的具体内容。

重点短语单词解析如下

(1)problem after problem 一个又一个的难题

(2)not least of which 最大的问题/难题

(3)deplete  vt. 耗尽/用尽/使枯竭

(4)which代替前面先行词"problem after problem"

(5)of which 属于“部分/个体与整体之间的关系

(6)least是“little”最高级别意思"最小的; 最少的; 最不重要的，地位最低的"这是最代词用法。

(7)no least of them=the biggest problem of them

(8)overcome  vt.克服；控制；战胜

特别注意：当1个名词在一个句子第2词出现的时候必须是用“代词”替换避免重复啊！!! 英语是一门不喜欢重复的语言，因此英语中会有很多种代词。