Draw the line at regulation (4)
I believe the answer is:
rule
I believe this is a triple definition!
'draw' is the first definition.
(ruling is a kind of drawing)
'the line' is the second definition.
(I've seen this in another clue)
'at' acts as a link.
'regulation' is the third definition.
(I know that regulation can be written as rule)
(Other definitions for rule that I've seen before include "Govern or control" , "Govern the straight edge?" , "Government - measure" , "Exercise authority over" , "Reign - law" .)