Realm of a top domestic? (7)

I believe the answer is:

kingdom

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'realm' is the definition.
(I know that realm can be written as kingdom)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for kingdom that I've seen before include "For one animal" , "Domain of a monarch" , "What Richard III would have given for a horse" , "All the animals" , "big territory?" .)

I've seen this clue in the Evening Standard.

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