Ancient trade route composed of dark soil (4,4)
I believe the answer is:
silk road
'ancient trade route' is the definition.
'composed of dark soil' is the wordplay.
'composed of' indicates anagramming the letters (the anagram is composed of the same letters).
'dark'+'soil'='darksoil'
'darksoil' with letters rearranged gives 'SILK ROAD'.
(Other definitions for silk road that I've seen before include "Caravan route" , "Old trade route to China" , "Ancient trade route from China" .)