Mysteriously, Quixote's creator set off to find underground chamber (6)

I believe the answer is:

cavern

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'underground chamber' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for cavern that I've seen before include "Subterranean chamber" , "Large dark hole in the ground" , "Detection device" , "vast enclosed space" , "Large underground chamber" .)

I've seen this clue in The Irish Times.

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