State in NATO organisation (6)
I believe the answer is:
nation
'state' is the definition.
(synonyms)
'in nato organisation' is the wordplay.
'organisation' is an anagram indicator.
'in'+'nato'='innato'
'innato' anagrammed gives 'NATION'.
(Other definitions for nation that I've seen before include "Country, state" , "Anoint (anag)" , "Turkey?" , "Large community in one place under one government" , "People of common culture or descent" .)