Head of poor country, a realm of yesteryear (7)

I believe the answer is:

prussia

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'a realm of yesteryear' is the definition.
I can't tell whether this definition defines the answer.

'head of poor country' is the wordplay.
'head of' says to take the initial letters.
'country' becomes 'russia' (Russia is a country).
The initial letter of 'poor' is 'p'.
'p'+'russia'='PRUSSIA'

Can you help me to learn more?

(Other definitions for prussia that I've seen before include "Old German kingdom, capital Berlin" , "Former German kingdom" , "It fought France, Eighteen-seventy" , "state before 1947" , "Powerful pre-1871 state of central Europe" .)

I've seen this clue in The Times.

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