Main thoroughfare used by merchants (5,5)
I believe the answer is:
trade route
'main' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for trade route that I've seen before include "Way followed by merchants" , "Caravans may follow it" , "path of traffic" , "Mercantile artery" .)