The most common and most likely answer to this crossword clue is the 7 letter word HILLMAN
I've seen this crossword clue in The Daily Telegraph
H
I
L
L
M
A
N
Buddy explains!

The straight part
We know that "Old car" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old car = hillman.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 HILLMAN mean?
HILLMAN noun- A native or inhabitant of hilly or mountainous country; a tribesman who lives in the mountains.