One on a trail of coarse mixed stone (6)
I believe the answer is:
dogger
I believe this is a double definition.
'one on a trail' is the first definition.
(to dog can mean to follow or trail)
'coarse mixed stone' is the second definition.
'of' is the link.
(Other definitions for dogger that I've seen before include "Two-masted fishing boat" , "A bank" , "Shipping forecast area" , "one that is continuously troublesome" .)