Old kingdom’s power over large country (7)
I believe the answer is:
prussia
'old kingdom's' is the definition.
(I've seen this before)
'power over large country' is the wordplay.
'power over' becomes 'p' (I've seen this in other clues).
'large country' becomes 'russia' (I've seen this before).
'p'+'russia'='PRUSSIA'
(Other definitions for prussia that I've seen before include "Former German state" , "Powerful pre-1871 state of central Europe" , "Former German kingdom" , "Leading German state from 1525 to 1945" , "old part of Europe" .)