The most common and most likely answer to this crossword clue is the 9 letter word SHORTHORN
I've seen this crossword clue in The Times
S
H
O
R
T
H
O
R
N
Buddy explains!
The straight part
We know that "stock item" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stock item = shorthorn.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 SHORTHORN mean?
SHORTHORN noun- One of a breed of cattle, originating in England, with distinctively short horns (in contrast to longhorn cattle).
- Describing cattle that have distinctively short horns.