The most common and most likely answer to this crossword clue is the 5 plus 5 letter word BRAKE PEDAL
B
R
A
K
E
P
E
D
A
L
Buddy explains!
The straight part
We know that "One stops a car" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one stops a car = brake pedal.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.