Grin with glee finally breaking rule (5)
I believe the answer is:
reign
'rule' is the definition.
(reign is a kind of rule)
'grin with glee finally breaking' is the wordplay.
'with' means one lot of letters go next to another.
'finally' says to take the final letters.
'breaking' indicates an anagram.
The last letter of 'glee' is 'e'.
'grin' after 'e' is 'egrin'.
'egrin' anagrammed gives 'REIGN'.
(Other definitions for reign that I've seen before include "Period of a monarchy" , "Dominion" , "Rule - dominate" , "Rule of monarch" , "Monarch's period" .)