Confine boiler (6)

I believe the answer is:

kettle

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'boiler' is the definition.
(I know that boiler can be written as kettle)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for kettle that I've seen before include "Vessel for boiling liquids" , "Kitchen container and type of drum" , "One boiled" , "Spouted vessel" , "Water boiler" .)

I've seen this clue in The Mirror.

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