It’s laid down (3)
I believe the answer is:
Here is my best explanation:
'down' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
I don't understand the remainder of the clue.
(Other definitions for law that I've seen before include "Body of rules of conduct for a society", "A rule", "Edict", "Set of rules laid down by authority", "Commandment", "Rule imposed by authority", "Statute", "Rule of conduct", "Collection of rules imposed by authority" and "Official rule".)