Old European state facing large country with power (7)
I believe the answer is:
prussia
'old european state' is the definition.
(I've seen this before)
'large country with power' is the wordplay.
'large country' becomes 'russia' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'power' becomes 'p' (symbol in physics).
'russia' put after 'p' is 'PRUSSIA'.
'facing' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for prussia that I've seen before include "state before 1947" , "Former kingdom, included northern Germany and northern Poland" , "Bismarck its PM" , "old part of Europe" , "Powerful pre-1871 state of central Europe" .)