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The only time one and two don't make three (6)

Ross

I believe the answer is:

twelve

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'three' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for twelve that I've seen before include "Highest number on clock" , "six couples" , "'Apostles, jury and months (6)'" , "Lunchtime" , "Midday" .)

I've seen this clue in the Evening Standard.
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