The most common and most likely answer to this crossword clue is the 6 letter word CABMAN
I've seen this crossword clue in The Guardian
C
A
B
M
A
N
More clues leading to the result CABMAN
Buddy explains!

The straight part
We know that "He once drove" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case he once drove = cabman.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 CABMAN mean?
CABMAN noun- The driver of a hackney cab.
- The driver of a taxi.