Etonian forced to change Union (13)
I believe the answer is:
confederation
'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.
'etonian'+'forced'='etonianforced'
'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" .)