The most common and most likely answer to this crossword clue is the 7 letter word BANGERS
I've seen this crossword clue in The Daily Telegraph
B
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More clues leading to the result BANGERS
Buddy explains!

The straight part
We know that "Cars that may go with 15?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cars that may go with 15? = bangers.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 BANGERS mean?
BANGERS noun- A thing or person which bangs, in any sense.
- : A firework that makes a bang.
- A woman's breast.
- A sausage.
- An old, worn-out car.
- A cylinder in a car.
- A powerfully energetic piece of music, especially dance music.
- (by extension) Any particularly good or pleasing thing.
- Someone who indulges in group sex -- see gangbang.
- A member of a violent gang.
- A violent person.