Should be green, but you can arrange to get it in walnut (4)
I believe the answer is:
lawn
'walnut' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for lawn that I've seen before include "Fine cotton or smooth grass" , "Cultivated and mowed grass" , "Garden feature" , "material" , "Fine fabric" .)