Crossword Genius

At snooker, they aren't so good as the blacks (4)

Ross

I believe the answer is:

reds

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'at snooker' is the definition.
Although both the answer and 'snooker' are nouns, they are not in the same form of the noun. However, I know that 'snooker' relates to this answer.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for reds that I've seen before include "Colours for communists" , "Scarlet and crimson, say" , "Tidies up" , "Colourful communists" , "First snooker balls to be potted" .)

I've seen this clue in the Evening Standard.
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