Let off steam about European border in earnest (8)
I believe the answer is:
vehement
'in earnest' is the definition.
(thesaurus)
'let off steam about european border' is the wordplay.
'let off steam' becomes 'vent' (both can mean to release anger).
'about' is an insertion indicator.
'european' becomes 'E' (abbreviation e.g. EU).
'border' becomes 'hem' (I've seen this before).
'e'+'hem'='ehem'
'vent' going around 'ehem' is 'VEHEMENT'.
(Other definitions for vehement that I've seen before include "Vigorous, emphatic" , "Intensely forceful" , "Emphatic, intense" , "Showing strong feeling, intense in speech" , "Burning" .)