Kingdom's queen protected by 1000 agents (6)

I believe the answer is:

mercia

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'kingdom's' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I don't see?

'queen protected by 1000 agents' is the wordplay.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'protected by' is an insertion indicator.
'1000' becomes 'M' (Roman numeral).
'agents' becomes 'CIA' (Central Intelligence Agency).
'm'+'cia'='mcia'
'er' going into 'mcia' is 'MERCIA'.

Can you help me to learn more?

(Other definitions for mercia that I've seen before include "Old English kingdom, once ruled by Offa" , "Kingdom of Middle England" , "place of Lady Godiva" , "share of Albion?" , "Ancient British kingdom" .)

I've seen this clue in The Times.

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