Oppose all others across South Island (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'all others across south island' is the wordplay.
'all others' becomes 'rest' (as in the rest of a group).
'across' means one lot of letters goes inside another.
'south' becomes 's' ('S' can be a synonym of 'south').
'island' becomes 'i'.
's'+'i'='si'
'rest' going around 'si' is 'RESIST'.
(Other definitions for resist that I've seen before include "Use force to prevent something from happening to sister" , "Withstand, struggle against" , "Fight back" , "Hold out for sister" , "Refuse to give in to" .)