5 across
That
island
repeatedly
disturbed
this
island
(6)
That island repeatedly disturbed this island (6)
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The definition is at the end of the clue.
How the clue works
The definition is this island, pointing to the well-known Pacific island. You take THAT and the letter I for 'island' twice ('repeatedly'), then 'disturbed' signals an anagram of those six letters — T, H, A, T, I, I — which rearranges neatly into the answer.