King George accepts that is revolutionary new rule (5)
I believe the answer is:
reign
'rule' is the definition.
(reign is a kind of rule)
'king george accepts that is revolutionary new' is the wordplay.
'king george' becomes 'gr' (Georgius Rex).
'accepts' is an insertion indicator.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'revolutionary' shows that the letters should be reversed in order.
'new' becomes 'n'.
'gr' placed around 'ie' is 'gier'.
'gier' back-to-front is 'reig'.
'reig'+'n'='REIGN'
(Other definitions for reign that I've seen before include "Have sovereign power" , "Rule as king or queen" , "time in power" , "Period on the throne" , "Monarch's rule" .)