European Union supporter seizing opening at last (10)
I believe the answer is:
eventually
'last' is the definition.
(thesaurus)
'european union supporter seizing opening at' is the wordplay.
'european union' becomes 'eu'.
'supporter' becomes 'ally' (I've seen this before).
'seizing' is an insertion indicator.
'opening at' becomes 'vent' (I am not sure about this - if you are sure you should believe this answer much more).
'eu'+'ally'='eually'
'eually' placed around 'vent' is 'EVENTUALLY'.
Can you help me to learn more?
(Other definitions for eventually that I've seen before include "sooner or later" , "In the fullness of time" , "At last" , "In due course" , "In the end" .)