Leaders from every nation happy to include new country (7)
I believe the answer is:
england
'country' is the definition.
(England is an example)
'leaders from every nation happy to include new' is the wordplay.
'leaders from' says to take the initial letters.
'happy' becomes 'glad' ('glad' can be similar in meaning to 'happy'**).
'to include' means one lot of letters goes inside another.
'new' becomes 'n'.
The first letters of 'every nation' is 'en'.
'glad' going around 'n' is 'gland'.
'en'+'gland'='ENGLAND'
(Other definitions for england that I've seen before include "Vera Lynn was sure there would always be one" , "Largest country in the UK" , "Lend nag (anag)" , "European country" , "... and its World Cup holders" .)