Game was first subjected to laws (5)
I believe the answer is:
ruled
'subjected to laws' is the definition.
(ruled by laws)
'game was first' is the wordplay.
'game' becomes 'RU' (rugby union).
'was first' becomes 'led'.
'ru'+'led'='RULED'
(Other definitions for ruled that I've seen before include "Governed or commanded" , "Exercised control over" , "Reigned or ordered" , "Were tops" , "Made authoritative statement" .)