Islander certain to move when island disappears (6)

I believe the answer is:

cretan

'islander' is the definition.
(someone from Crete)

I cannot understand how the remainder of the clue works.

(Other definitions for cretan that I've seen before include "Native of Minoan island" , "From Greece" , "Of a Greek island, centre of the Minoan civilisation" , "One comes from a Mediterranean island" , "Native of a large Greek island" .)

I've seen this clue in The Guardian.

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