Crossword Genius

Take an age to score a single (6-3)

Ross

I believe the answer is:

twenty-one

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'take an age' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'score a single' is the wordplay.
'score' becomes 'twenty' (synonyms).
'a single' becomes 'one' (synonyms).
'twenty'+'one'='TWENTY-ONE'

'to' acts as a link.

Can you help me to learn more?

(Other definitions for twenty-one that I've seen before include "XXI - used to be the coming of age year" , "Card game, blackjack or pontoon" , "Target total in blackjack or pontoon" .)

I've seen this clue in The Guardian.
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