Historic period, as a rule (7)
I believe the answer is:
regency
This could be a double definition.
'historic period' is the first definition.
'Regency' can be an answer for 'period' (Regency is an example). I am not sure about the 'historic' bit.
'a rule' is the second definition.
(I know that regency is a type of rule)
'as' acts as a link.
Can you help me to learn more?
(Other definitions for regency that I've seen before include "Periods of the Madness of King George when the Prince of Wales took over" , "British period, Eighteen-Eleven to Eighteen-Twenty" , "Period (1811-20) covering The Madness of King George III" , "Period of 1811 - 20" , "Period of king's incapacity" .)