Supreme power (11)
(Other definitions for sovereignty that I've seen before include "Pre-eminence" , "it rules!" , "Self-government" .)
'supreme power' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for empire that I've seen before include "Number of countries ruled by one" , "Group of states under the control one authority" , "Multinational" , "Great power" , "Huge domain or group of companies" .)