The most common and most likely answer to this crossword clue is the 9 letter word GREENSAND
G
R
E
E
N
S
A
N
D
Buddy explains!
The straight part
We know that "running across Kent and Surrey" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case running across kent and surrey = greensand.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 GREENSAND mean?
GREENSAND noun- A greenish sandstone containing glauconite.