Marriage is with Democrat in the end, not Republican (8)
I believe the answer is:
unionist
'not republican' is the definition.
(I've seen this before)
'marriage is with democrat in the end' is the wordplay.
'marriage' becomes 'union' (synonyms).
'with' says to put letters next to each other.
'in the end' says to take the final letters.
The final letter of 'democrat' is 't'.
'union'+'is'+'t'='UNIONIST'
(Other definitions for unionist that I've seen before include "Supporter of the cohesion of England, Wales, Scotland and Northern Ireland" , "Civil warmonger" , "not Republican" , "One doesn't want to break with Britain" , "I favour joint enterprise?" .)