Alliance including one new county (6)
I believe the answer is:
nation
'county' is the definition.
(thesaurus)
'alliance including one new' is the wordplay.
'alliance' becomes 'nato' (NATO is an example).
'including' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'nato' placed around 'i' is 'natio'.
'natio'+'n'='NATION'
(Other definitions for nation that I've seen before include "Large community in one place under one government" , "Country, state" , "People of a country" , "Body of people under one government" , "Race" .)