The grass one is of marginal interest (5)
I believe the answer is:
verge
'the grass' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for verge that I've seen before include "Grass edge to road" , "Grassy road edge" , "Side of road" , "Margin or brink" , "emblem of office for bishop" .)