Among others, one’s up to oppose (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'among others one's up' is the wordplay.
'among' means one lot of letters goes inside another (I've seen this in other clues).
'others' becomes 'rest' (I've seen this before).
'one's' becomes 'I's' (Roman numeral).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'is' back-to-front is 'si'.
'rest' placed around 'si' is 'RESIST'.
'to' is the link.
(Other definitions for resist that I've seen before include "Don't give in" , "Oppose or fight against someone" , "Oppose, put up defence" , "Stand firm against" , "Use force to prevent something from happening to sister" .)