Etonian forced to change Union (13)
I believe the answer is:
'union' is the definition.
(I know that confederation is a type of coalition)
'etonian forced to change' is the wordplay.
'to change' indicates an anagram.
'etonianforced' is an anagram of 'CONFEDERATION'.
(Other definitions for confederation that I've seen before include "Group of allies" , "Union (of states)" , "Alliance of states" , "League of states" , "Loose alliance of political units" .)