The realm of Isambard Brunel (7)
I believe the answer is:
kingdom
'the realm' is the definition.
(thesaurus)
I cannot 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 "All the animals" , "Domain of royal ruler" , "Monarchical state" , "Regal realm" , "Domain of a monarch" .)