Domain of Isambard Brunel (7)
I believe the answer is:
kingdom
'domain' is the definition.
(I know that kingdom is a type of domain)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for kingdom that I've seen before include "For one animal" , "State with a monarchy" , "What Richard III would have given for a horse" , "big territory?" , "Regal realm" .)