Everybody that is can change in union (8)
I believe the answer is:
alliance
'union' is the definition.
(alliance is a kind of union)
'everybody that is can change in' is the wordplay.
'everybody' becomes 'all' (I've seen this in another clue).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'change' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'can' with letters rearranged gives 'anc'.
'ie' enclosing 'anc' is 'iance'.
'all'+'iance'='ALLIANCE'
(Other definitions for alliance that I've seen before include "Union, association" , "Union for mutual benefit" , "Confederation" , "Formal agreement" , "Coalition or union" .)