The most common and most likely answer to this crossword clue is the 10 letter word CONTAGIOUS
I've seen this crossword clue in The Daily Telegraph
C
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Buddy explains!

The straight part
We know that "12" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 12 = contagious.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 CONTAGIOUS mean?
CONTAGIOUS adjective- (of a disease) Easily transmitted to others.
- Easily passed on to others.
- (of a person) Having a disease that can be transmitted to another person.