A case of one and two not making three (6)

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.
Maybe they are linked in a way I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for twelve that I've seen before include "Six twos" , "Half the number here" , "Midnight or noon" , "[MIDNIGHT], perhaps" , "Number of Apostles" .)

I've seen this clue in the Evening Standard.

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