The most common and most likely answer to this crossword clue is the 13 letter word SERENDIPITOUS
I've seen this crossword clue in The Sunday Telegraph
S
E
R
E
N
D
I
P
I
T
O
U
S
More clues leading to the result SERENDIPITOUS
- President’s IOU written off? That’s fortunate
- See disruption unfortunately discovered by accident
- Discovered by chance top EU insiders intoxicated
- Chance used to inspire anagrams
- Luckily found our destinies transformed, seizing power
- Stop near edge of immense chasm to enter one abandoned grave discovered by chance
Buddy explains!

The straight part
We know that "Luckily found" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case luckily found = serendipitous.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 SERENDIPITOUS mean?
SERENDIPITOUS adjective- By serendipity; by unexpected good fortune
- Good, beneficial, favorable