The most common and most likely answer to this crossword clue is the 7 letter word JURYMAN
I've seen this crossword clue in The Guardian
J
U
R
Y
M
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More clues leading to the result JURYMAN
Buddy explains!

The straight part
We know that "One of a dozen in a box, in case" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of a dozen in a box, in case = juryman.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 JURYMAN mean?
JURYMAN noun- One who is impaneled on a jury, or who serves as a juror.