Take an age to score a single (6-3)
I believe the answer is:
'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).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for twenty-one that I've seen before include "Target total in blackjack or pontoon", "Card game, blackjack or pontoon", "XXI - used to be the coming of age year".)