The most common and most likely answer to this crossword clue is the 7 plus 13 letter word POMPEII (HERCULANEUM)
I've seen this crossword clue in The Guardian
P
O
M
P
E
I
I
(
H
E
R
C
U
L
A
N
E
U
M
)
Buddy explains!
The straight part
We know that "Ruined city" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ruined city = pompeii (herculaneum).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.