One who laid down the law: Grandma, possibly? (5)
I believe the answer is:
moses
This clue is a double definition.
'one who laid down the law' is the first definition.
(Moses received the Ten Commandments)
'grandma possibly?' is the second definition.
(painter Grandma Moses)
(Other definitions for moses that I've seen before include "Leader of Jewish Exodus" , "the tablet-taker?" , "He parted the Red Sea" , "Prophet of the Old Testament" , "He led the Jews towards the Promised Land" .)