as to=about=concerning 关于，对于

Local inhabitants haven'tthe slightest doubt as to who is the rightful owner.  

当地居民对谁是合法所有者没有丝毫疑问。

He is in no doubt as to what is needed.  

他很确定需要什么。

We have found some drivers clueless as to the law.  

我们发现有些司机对这一法规一无所知。

句子结构分析如下

To this day（时间状语）,// no clues（主语）// have been found （谓语）//（修饰主语后置定语）as to //（宾语从句）what happened to this wealthy American businessman //（定语从句）who is credited with single-handedly reviving the Thai silk industry.

参考译文：直到今天，还没有发现关于这位富有的美国商人发生了什么事情的线索，他被认为是一手重振泰国丝绸产业的人。

重点结构解析如下

(1)to this day为句子时间状语

(2)no clues为句子主语

(3)have been found为句子谓语

(4)as to what happened to this wealth American business...为主语no clues后置定语

(5)what happened to this wealthy American businessman..为as to的宾语从句

(6)who is credited with single-handedly reviving the Thai silk industry.为名词“this wealthy American businessman”后置定语(who为关系代词引导定语从句并在定语从句作主语成分）

To this day, no clues have been found as to what happened to this wealthy American businessman who is credited with single-handedly reviving the Thai silk industry.

把上面句子语序调整后为面句子

=To this day（时间状语）, no clues（主语）//（主语后置定语） as to what happened to this wealthy American businessman who is credited with single-handedly reviving the Thai silk industry //（谓语）have been found