Old kingdom with power over large country (7)
I believe the answer is:
prussia
'old kingdom' is the definition.
(I've seen this before)
'with power over large country' is the wordplay.
'with' is a charade indicator (letters next to each other).
'power over' becomes 'p' (I've seen this in other clues).
'large country' becomes 'russia' (I've seen this before).
'p' put next to 'russia' is 'PRUSSIA'.
(Other definitions for prussia that I've seen before include "Former kingdom, included northern Germany and northern Poland" , "Old part of Germany" , "Powerful pre-1871 state of central Europe" , "Former German state" , "Old German kingdom, capital Berlin" .)