After the start of expansion Nato is changing this country (7)
I believe the answer is:
estonia
'this country' is the definition.
(I've seen this before)
'after the start of expansion nato is changing' is the wordplay.
'after' is a charade indicator (letters next to each other).
'the start of' suggests taking the first letters.
'changing' indicates an anagram.
The first letter of 'expansion' is 'e'.
'nato'+'is'='natois'
'natois' anagrammed gives 'stonia'.
'e' put next to 'stonia' is 'ESTONIA'.
(Other definitions for estonia that I've seen before include "Country due S. of Finland" , "Republic in northeastern Europe" , "Northern European country" , "European republic, became independent again in 1991" , "One of the Baltic States" .)