Island hostile to Guam, for the most part (7)
I believe the answer is:
antigua
'island' is the definition.
(I know that Antigua is a island)
'hostile to guam for the most part' is the wordplay.
'hostile to' becomes 'anti' (anti means hostile to or opposed to).
'for the most part' means to remove the last letter (most but not all of the letters).
'guam' with its last letter taken off is 'gua'.
'anti'+'gua'='ANTIGUA'
(Other definitions for antigua that I've seen before include "One of the Leeward Islands" , "Aga unit in the West Indies" , "former British possession" , "Largest of Leeward Is." , "Caribbean island linked with Barbuda" .)