The most common and most likely answer to this crossword clue is the 4 letter word MERC
I've seen this crossword clue in The Guardian
M
E
R
C
More clues leading to the result MERC
- Some wiseacre moaning about posh car
- Posh car seen in commercial
- Free companion over a note set alight
- Picked up dark vehicle, briefly
- One who’s paid in military emergency requiring cool heads
- Band retired with Charlie in limo
- Hired soldier as host to welcome queen
- “Danke” translated almost as “German vehicle”
- Soldier‘s claim to be religious?
- Luxury car kept in warmer climes
Buddy explains!

The straight part
We know that "vehicle, briefly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vehicle, briefly = merc.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 MERC mean?
MERC noun- A mercenary.