Decree on new grass (4)
I believe the answer is:
lawn
'grass' is the definition.
(a lawn is an area of grass)
'decree on new' is the wordplay.
'decree' becomes 'law' (I've seen this in another clue).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'law'+'n'='LAWN'
(Other definitions for lawn that I've seen before include "Area of smooth grass - fine linen" , "Fine cotton or smooth grass" , "Blades trimmed" , "Linen fabric" , "Cultivated and mowed grass" .)