Policy that may be laid down by ruler (4)
I believe the answer is:
line
I believe this clue is a double definition.
'policy' is the first definition.
(as in the party line)
'that may be laid down by ruler' is the second definition.
(you might draw a line using a ruler)
(Other definitions for line that I've seen before include "Wrinkle; phone link" , "business" , "Row - occupation" , "Long mark; direction" , "Fill" .)