He's in the best rebuilt old place (6)
I believe the answer is:
thebes
'old place' is the definition.
The answer and definition can be both locations as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'he's in the best rebuilt' is the wordplay.
'in' indicates putting letters inside.
'rebuilt' indicates an anagram.
'best' with letters rearranged gives 'tbes'.
'he' going inside 'tbes' is 'THEBES'.
Can you help me to learn more?
(Other definitions for thebes that I've seen before include "Old Egyptian city" , "Ancient city, a rival to Athens" , "One of the oldest Greek cities" , "Old city of Greece or Egypt" , "Ancient Egyptian or Greek city" .)