Cryptic crossword clue
Shakespearean fool joins gang, becoming rotten 9

The most common and most likely answer to this crossword clue is the 9 letter word FESTERING

I've seen this crossword clue in The Guardian
F
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Buddy explains!

The straight part
We know that "becoming rotten /becoming rotten" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case becoming rotten /becoming rotten = festering.
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 FESTERING mean?

FESTERING verb
  1. To become septic; to become rotten.
  2. To worsen, especially due to lack of attention.
  3. To cause to fester or rankle.
FESTERING noun
  1. The condition of something that festers.

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