Country where native leader is seen in Roman costume (5)

I believe the answer is:

tonga

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

I can't explain the rest of the clue.

(Other definitions for tonga that I've seen before include "Polynesian island" , "Tropic island" , "Islands once ruled by Queen Salote" , "SW Pacific monarchy" , "South Pacific island group country" .)

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