Island egg shelled, so to speak? (5)

I believe the answer is:

slang

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'to speak?' is the definition.
(I know that slang is a more specific form of the action speak)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for slang that I've seen before include "Be abusive" , "colloquialisms" , "Unconventional language" , "Vulgar language" , "Argot" .)

I've seen this clue in The Guardian.

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