It's hard, running along the motorway! (8)
I believe the answer is:
shoulder
'motorway' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for shoulder that I've seen before include "Joint - ours held (anag)" , "On the side of the road, it might be hard" , "Part of the body (that bears burdens?)" , "Undertake (burden)" , "See 10" .)