The most common and most likely answer to this crossword clue is the 7 plus 2 plus 6 letter word STREETS OF LONDON
I've seen this crossword clue in Financial Times
S
T
R
E
E
T
S
O
F
L
O
N
D
O
N
Buddy explains!
The straight part
We know that "Number of ways to get around a big city" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case number of ways to get around a big city = streets of london.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.