Oppose remainder crossing South Island? (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'remainder crossing south island?' is the wordplay.
'remainder' becomes 'rest' (synonyms).
'crossing' indicates putting letters inside.
'south' becomes 'S' (abbreviation).
'island?' becomes 'i' (geographical abbreviation).
's'+'i'='si'
'rest' placed around 'si' is 'RESIST'.
(Other definitions for resist that I've seen before include "Put up fight, oppose forcefully" , "'Withstand, fight against (6)'" , "Endure" , "Put up opposition to sister" , "Bar" .)