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Rattling gaoler chap to keep one, in the main, studded with keys (11)

Ross

I believe the answer is:

archipelago

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But I can't explain it at all! Could you help me understand?

(Other definitions for archipelago that I've seen before include "Fiji, perhaps" , "Large group of islands" , "Group of many islands" , "Cluster of islands" , "Extensive group of islands" .)

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