Leading European state (5)
I believe the answer is:
maine
'state' is the definition.
(Maine is an example)
'leading european' is the wordplay.
'leading' becomes 'main'.
'european' becomes 'e' (abbreviation e.g. EU).
'main'+'e'='MAINE'
(Other definitions for maine that I've seen before include "state of USA" , "The USA's Pine Tree State" , "New England state" , "American state, capital Augusta" , "somewhere in America" .)