Once a year idealists leave Aleutian Islands (6)

I believe the answer is:

annual

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'once a year' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for annual that I've seen before include "yearly event" , "Book; occurring once a year" , "Plant that lives and dies within one year" , "Occuring once a year" , "Hardy it may be" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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