The most common and most likely answer to this crossword clue is the 9 letter word POSTILION
I've seen this crossword clue in The Guardian
P
O
S
T
I
L
I
O
N
More clues leading to the result POSTILION
Buddy explains!
The straight part
We know that "carriage guide" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case carriage guide = postilion.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 POSTILION mean?
POSTILION noun- A rider mounted on the near (left) leading horse who guides the team pulling a carriage.
- A post-boy, a messenger boy, a swift letter carrier.