Most important European state? (5)
I believe the answer is:
maine
'state?' is the definition.
(Maine is an example)
'most important european' is the wordplay.
'most important' becomes 'main' (I've seen this before).
'european' becomes 'e' (abbreviation e.g. EU).
'main'+'e'='MAINE'
(Other definitions for maine that I've seen before include "American state, capital Augusta" , "The USA's Pine Tree State" , "N. US Atlantic state" , "NE US state" , "US state, capital Augusta (5)" .)