Forceful opening across European border (8)
I believe the answer is:
vehement
'forceful' is the definition.
(thesaurus)
'opening across european border' is the wordplay.
'opening' becomes 'vent' (vent is a kind of opening**).
'across' indicates putting letters inside.
'european' becomes 'e' (abbreviation e.g. EU).
'border' becomes 'hem' (I've seen this before).
'e'+'hem'='ehem'
'vent' placed around 'ehem' is 'VEHEMENT'.
(Other definitions for vehement that I've seen before include "Violent" , "Forceful, ardent" , "Intensely forceful" , "Furious" , "Irish symbol" .)