Run off with reactionary East European (5)
I believe the answer is:
elope
'run off' is the definition.
(I know that run off can be written as elope)
'reactionary east european' is the wordplay.
'reactionary' says the letters should be written backwards (reactions happen in the opposite direction).
'east' becomes 'e' (abbreviation).
'european' becomes 'pole' (someone from Poland).
'e'+'pole'='epole'
'epole' in reverse letter order is 'ELOPE'.
'with' is the link.
(Other definitions for elope that I've seen before include "Run away secretly to marry" , "(Lovers) run away" , "escape" , "run off with another?" , "Abscond for amorous purposes" .)