网友的说法正确。答案说的“after 引导的宾从” 错误。应该这样描述：

after 是介词，后面的介词宾语是由从句充当，介词短语作时间状语。而不是after 引导的宾从。

我认为，问题的难点不在于叫什么名称，而是在于本句为什么用what ，而不用 it，that，there 作主语。

After _____ seemed an endless wait, it was his turn to enter the personal office.

我给网友提供三种理解方式：

1. 这里，afer 是介词，不是连词。后面的成分是介宾，由名词性从句充当。what seemed an endless wait，其本质是一个“时间概念”的名词性短语，相当于 three hours, a long time。可以替换试试：

After what seemed an endless wait, it was his turn to enter the personal office.

After three hours/a long time, it was his turn to enter the personal office.

2. 此外，我们还可以把 what 分解。这里，what 是“融合性关系代词”，等于 the wait that...

After the wait that seemed an endless one（定语从句）, it was his turn to enter the personal office.

3. 还有一种简单的理解办法，因为 what 和 an endless wait 是“主表关系”，即它们是等同的两个概念，what = an endless wait，这样，去掉主语和系动词，只保留表语，句子仍然正确：

After an endless wait, it was his turn to enter the personal office.