Oppose resolution on island intersection (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'resolution on island intersection' is the wordplay.
'resolution' becomes 'res' (I've seen this in other clues).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'island' becomes 'is' (geographical abbreviation).
'intersection' becomes 'T' (T junction).
'res'+'is'+'t'='RESIST'
(Other definitions for resist that I've seen before include "Oppose, put up defence" , "Withstand, oppose" , "Stand up to, fight against" , "Hold out for sister" , "Go against" .)