The most common and most likely answer to this crossword clue is the 9 letter word RUINATION
I've seen this crossword clue in The Daily Telegraph
R
U
I
N
A
T
I
O
N
More clues leading to the result RUINATION
- ... undoing, going with mounting resistance
- Discharge of water runs to the west, leading to disaster
- Are you texting people fed up with one’s downfall?
- Downfall of one country after old city’s revolution
- Breakdown out in Iran, unfortunately
- Damage pants out in rain
- Wreckage may be acceptable in cutting allotment
- … helping to keep union oddly in breakdown
- Damage from driving out in rain
- Passing water leaders swap places causing devastation
Buddy explains!

The straight part
We know that "Damage" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case damage = ruination.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 RUINATION mean?
RUINATION noun- The state of being ruined, a state of devastation or destruction.
- The act of ruining or wrecking.
- The cause of being ruined, destroyed or lost.
- A loss of reputation.