Some assemblymen ending up in opposition (5)
I believe the answer is:
enemy
'opposition' is the definition.
('enemy' can be a synonym of 'opposition')
'some assemblymen ending up' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ymene' is hidden within 'assemblymen ending'.
'ymene' back-to-front is 'ENEMY'.
'in' is the link.
(Other definitions for enemy that I've seen before include "Time" , "is against us" , "Antagonist" , "Yemen (anag.)" , "Opposing force" .)