Rule, new, for grassy area (4)
I believe the answer is:
lawn
'grassy area' is the definition.
(I've seen this before)
'rule new' is the wordplay.
'rule' becomes 'law' (law is a kind of rule).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'law'+'n'='LAWN'
'for' is the link.
(Other definitions for lawn that I've seen before include "Area of mown grass" , "Grass-covered ground" , "Smooth grass area, often in front of house" , "Expanse of grass" , "Fine woven cotton" .)