Stop affair with European (5)
I believe the answer is:
cease
'stop' is the definition.
(synonyms)
'affair with european' is the wordplay.
'affair' becomes 'case'.
'with' is an insertion indicator.
'european' becomes 'e' (abbreviation e.g. EU).
'case' enclosing 'e' is 'CEASE'.
(Other definitions for cease that I've seen before include "Discontinue" , "Desist" , "Culminate" , "Stop, terminate" , "Stop, finish" .)