Ancient people mostly ready for Americans to occupy islands (5)

I believe the answer is:

iceni

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'islands' is the definition.
Both the answer and definition are plural nouns.
Maybe they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for iceni that I've seen before include "British people once" , "Boadicea's tribe" , "Very old folk" , "people rebelling against Rome" , "Old people" .)

I've seen this clue in The Telegraph.

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