Laid down the law (5)
I believe the answer is:
ruled
'laid down the law' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ruled that I've seen before include "Governed or commanded" , "Governed - with lines" , "Reigned - lined - decided" , "Exercised control over" , "Reigned or ordered" .)