Prime arrangement with eastern realm (6)
I believe the answer is:
empire
'realm' is the definition.
(thesaurus)
'prime arrangement with eastern' is the wordplay.
'arrangement' indicates anagramming the letters.
'with' says to put letters next to each other.
'eastern' becomes 'e' (**).
'prime' is an anagram of 'empir'.
'empir'+'e'='EMPIRE'
(Other definitions for empire that I've seen before include "Group of states under the same sovereign" , "Jurisdiction" , "In state of New York" , "Supreme sovereignty" , "Large realm" .)