Sentence:  To play the <WSD>postilion</WSD> is not an easy thing. It is a trade by itself—half a gunner's and half a groom's. It has to do with horses—that is bad enough ; but also it involves some knowledge of the road. 
Meaning of the word marked with <WSD> tags:  A rider mounted on the near (left) leading horse who guides the team pulling a carriage.