One island against another? Not entirely (7)
I believe the answer is:
'one island' is the definition.
(I've seen this before)
'against another? not entirely' is the wordplay.
I cannot really see how this works, but
'against' could be 'anti' (prefix meaning against) and 'anti' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for antigua that I've seen before include "Aga unit in the West Indies" , "Largest of Leeward Is." , "warm island" , "Caribbean island linked with Barbuda" , "One of the Leeward Islands" .)