Muslim land in east joining assembly of UN at last (9)
I believe the answer is:
sultanate
'muslim land' is the definition.
(country ruled by a sultan, a Muslim ruler)
'east joining assembly of un at last' is the wordplay.
'east' becomes 'e' (synonyms).
'joining' means one lot of letters go next to another.
'assembly of' is an anagram indicator.
'un'+'at'+'last'='unatlast'
'unatlast' anagrammed gives 'sultanat'.
'e' put after 'sultanat' is 'SULTANATE'.
'in' acts as a link.
(Other definitions for sultanate that I've seen before include "Turkey was" , "Brunei?" , "Muslim dominion" , "Country" .)