Cryptic crossword clue
Previously known as Capone, evildoer’s imprisoned 3

The most common and most likely answer to this crossword clue is the 3 letter word NEE

I've seen this crossword clue in Financial Times
N
E
E


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The straight part
We know that "Previously known as" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case previously known as = nee.
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 NEE mean?

NEE adjective
  1. Used when giving the maiden name of a woman.
  2. Used when giving a former name. Originally known as.
NEE interjection
  1. No, used to express no as a quantity, i.e. not any, like German kein/Dutch geen/French rien. Compare with na.

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