The most common and most likely answer to this crossword clue is the 9 letter word JURYWOMAN
I've seen this crossword clue in The Guardian
J
U
R
Y
W
O
M
A
N
Buddy explains!
The straight part
We know that "She decides on suit, one of a dozen in box" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case she decides on suit, one of a dozen in box = jurywoman.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 JURYWOMAN mean?
JURYWOMAN noun- A female juror.