The most common and most likely answer to this crossword clue is the 10 letter word BREASTBONE
I've seen this crossword clue in The Guardian
B
R
E
A
S
T
B
O
N
E
Buddy explains!

The straight part
We know that "Item in trunk" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case item in trunk = breastbone.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 BREASTBONE mean?
BREASTBONE noun- The central narrow bone in the front of the chest, connecting the collarbone and the top ribs.