English and European in retreat flee together (5)
I believe the answer is:
elope
'flee together' is the definition.
'english and european in retreat' is the wordplay.
'english' becomes 'e' (abbreviation).
'and' means one lot of letters go next to another.
'european' becomes 'pole' (someone from Poland).
'in retreat' says the letters should be written backwards.
'pole' back-to-front is 'elop'.
'e' put after 'elop' is 'ELOPE'.
(Other definitions for elope that I've seen before include "(Lovers) run away" , "Do a runner" , "run off with another?" , "Run off with Mr/Ms Right" , "Run away with lover" .)