1000kg silverback in island country (5)

I believe the answer is:

tonga

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'island country' is the definition.
(I know that Tonga is a island country)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tonga that I've seen before include "Pacific island kingdom" , "Two-wheeled Indian vehicle" , "Polynesian monarchy, member of the Commonwealth" , "Islands once ruled by Queen Salote" , "part of Polynesia" .)

I've seen this clue in the Sydney Morning Herald.

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