Crossword Genius

Bad mood: one brought down with others? (6)

Ross

I believe the answer is:

domino

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for domino that I've seen before include "Small black block with 0 to 6 white dots" , "Black and white game piece" , "Black tile with white dots" , "one coming down with many others?" , "Rectangular block used in game" .)

I've seen this clue in The Sun.
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