The realm of Isambard Brunel (7)

I believe the answer is:

kingdom

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'the realm' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for kingdom that I've seen before include "All the animals" , "Domain of royal ruler" , "Monarchical state" , "Regal realm" , "Domain of a monarch" .)

I've seen this clue in the Evening Standard.

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