The most common and most likely answer to this crossword clue is the 7 letter word CENTURY
I've seen this crossword clue in The Times
C
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N
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Buddy explains!
The straight part
We know that "This is the 21st" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this is the 21st = century.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CENTURY mean?
CENTURY noun- A period of 100 consecutive years; often specifically a numbered period with conventional start and end dates, e.g., the twentieth century, which stretches from (strictly) 1901 through 2000, or (informally) 1900 through 1999. The first century AD was from 1 to 100.
- A unit in ancient Roman army, originally of 100 army soldiers as part of a cohort, later of more varied sizes (but typically containing 60 to 70 or 80) soldiers or other men (guards, police, firemen), commanded by a centurion.
- A political division of ancient Rome, meeting in the Centuriate Assembly.
- A hundred things of the same kind; a hundred.
- A hundred runs scored either by a single player in one innings, or by two players in a partnership.
- A score of one hundred points.
- A race a hundred units (as meters, kilometres, miles) in length.
- A banknote in the denomination of one hundred dollars.