Certain South Pacific needs a change of key (8)

I believe the answer is:

specific

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'certain' is the definition.
(thesaurus)

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

(Other definitions for specific that I've seen before include "Particular, definite" , "Particular, exact" , "Particular, special" , "Relating uniquely to a subject; precise, clear" , "Well-defined" .)

I've seen this clue in The Times.

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