One island against another, almost (7)
I believe the answer is:
antigua
'one island' is the definition.
(I know that Antigua is a island)
'against another almost' is the wordplay.
I cannot really understand how this works, but
'against' could be 'anti' (prefix meaning against) and 'anti' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for antigua that I've seen before include "Aga unit in the West Indies" , "West Indies island" , "former British possession" , "holiday destination" , "Largest of Leeward Is." .)