Plump condition caused by Christmas treat and drinks around rest centre (9)

I believe the answer is:

pudginess

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'plump' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

I can't explain the rest of the clue.

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(Other definitions for pudginess that I've seen before include "Chubbiness, plumpness" , "needing slimming down in this state" .)

I've seen this clue in the Sydney Morning Herald.

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