If it's not a white material it's green (4)
I believe the answer is:
'it's green' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for lawn that I've seen before include "Smooth grass garden in front of house" , "Fine cotton or smooth grass" , "Garden feature" , "Cultivated and mown grass" , "Fine fabric" .)