Attack from the rear to capture a Pacific island (4)
I believe the answer is:
Here is my best explanation:
'pacific island' is the definition.
'attack from the rear to capture a' is the subsidiary indication.
'attack' becomes 'mug' (**).
'from the rear' shows that the letters should be reversed in order.
'to capture' indicates putting letters inside.
'mug' reversed gives 'gum'.
'gum' going around 'a' is 'guam'.
(Other definitions for guam that I've seen before include "place in the Marianas", "US island east of the Philippines", "US unincorporated territory in the western Pacific", "Largest of the Mariana Islands", "NW Pacific island with large US base", "North Pacific island, base for the US navy and air force" and "Island in the West Pacific".)