The most common and most likely answer to this crossword clue is the 4 plus 7 letter word FORD LINCOLN
I've seen this crossword clue in The Guardian
F
O
R
D
L
I
N
C
O
L
N
Buddy explains!
The straight part
We know that "in one, JFK was shot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in one, jfk was shot = ford lincoln.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.