The most common and most likely answer to this crossword clue is the 8 plus 6 letter word CONCRETE JUNGLE
I've seen this crossword clue in The Daily Telegraph
C
O
N
C
R
E
T
E
J
U
N
G
L
E
More clues leading to the result CONCRETE JUNGLE
Buddy explains!
The straight part
We know that "Big city" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case big city = concrete jungle.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.