Undiagnosed without aid from those underlying cells (8)

I believe the answer is:

dungeons

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

I don't understand how the rest of the clue works.

(Other definitions for dungeons that I've seen before include "Underground cells for prisoners" , "Underground prisons" , "Underground prison cells" .)

I've seen this clue in the Irish Times.

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