Rule on new garden feature (4)
I believe the answer is:
lawn
'garden feature' is the definition.
(I've seen this before)
'rule on new' is the wordplay.
'rule' becomes 'law' (law is a kind of rule).
'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 "Fine cotton or smooth grass" , "Area of smooth grass - fine linen" , "Fine cotton or grass garden" , "Type of thin linen or cotton" , "Cultivated and mowed grass" .)