Crossword Genius

Unwanted topping of vitamin! Uncle's spitting out large coffee (6,3)

Ross

I believe the answer is:

dunces cap

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'coffee' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for dunces cap that I've seen before include "Ignorant boy's headgear" , "Classroom idiot's headgear" , "Headgear used to humiliate slow learners" .)

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