Finally become English Nationalist Party (3,2)
I believe the answer is:
end up
'finally become' is the definition.
(I've seen this in another clue)
'english nationalist party' is the wordplay.
'english' becomes 'e' (abbreviation).
'nationalist' becomes 'N' (abbreviation in politics).
'party' becomes 'dup' (Democratic Unionist Party).
'e'+'n'+'dup'='END UP'
(Other definitions for end up that I've seen before include "Come to rest" , "Turn out" , "finish ahead" , "head first" , "Eventually find oneself" .)