Everyone joining a nice new union (8)
I believe the answer is:
alliance
'union' is the definition.
(alliance is a kind of union)
'everyone joining a nice new' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'joining' is a charade indicator (letters next to each other).
'new' is an anagram indicator.
'a'+'nice'='anice'
'anice' anagrammed gives 'iance'.
'all'+'iance'='ALLIANCE'
(Other definitions for alliance that I've seen before include "Bond" , "Coalition or union" , "Federation" , "Confederation" , "League - treaty" .)