Prime arrangement with eastern realm (6)
I believe the answer is:
empire
'realm' is the definition.
(I've seen this in another clue)
'prime arrangement with eastern' is the wordplay.
'arrangement' indicates an anagram.
'with' means one lot of letters go next to another.
'eastern' becomes 'e' ().
'prime' anagrammed gives '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" .)