Crossword Genius

The old "one-two"? Pretty young, actually (6)

Ross

I believe the answer is:

twelve

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

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 "XII" , "houses in a year?" , "Six twos" , "Last hour" , "Lunchtime" .)

I've seen this clue in the Evening Standard.
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