Game is protected by trademark law (6)
I believe the answer is:
truism
'law' is the definition.
(thesaurus)
'game is protected by trademark' is the wordplay.
'game' becomes 'RU' (rugby union).
'protected by' indicates putting letters inside (inserted letters are held or protected).
'trademark' becomes 'tm' (abbreviation for trademark).
'ru'+'is'='ruis'
'ruis' placed into 'tm' is 'TRUISM'.
(Other definitions for truism that I've seen before include "Statement saying nothing new" , "Statement or fact of obvious veracity" , "An obvious truth" , "Bromide" , "self-evident statement" .)