Something healthy to eat during one's confinement? (8)

I believe the answer is:

porridge

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'confinement?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I don't understand the remainder of the clue.

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(Other definitions for porridge that I've seen before include "Breakfast dish or slangy prison term" , "Breakfast food - prison term" , "Look in the GPO drier for breakfast" , "Breakfast cereal dish" , "Oat dish; time in prison (slang)" .)

I've seen this clue in The Irish Times.

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