Sounds like someone ailing in the chest (6)

I believe the answer is:

coffer

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'chest' is the definition.
(I know that coffer is a type of chest)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for coffer that I've seen before include "Treasure chest" , "Strongbox" , "Money chest sounds like one with lung trouble" , "Small chest for keeping valuables" , "jewellery store?" .)

I've seen this clue in The Globe and Mail.

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