The most common and most likely answer to this crossword clue is the 8 letter word DESERTED
I've seen this crossword clue in The Daily Telegraph
D
E
S
E
R
T
E
D
More clues leading to the result DESERTED
Buddy explains!

The straight part
We know that "Walked out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case walked out = deserted.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 DESERTED mean?
DESERTED verb- To leave (anything that depends on one's presence to survive, exist, or succeed), especially when contrary to a promise or obligation; to abandon; to forsake.
- To leave one's duty or post, especially to leave a military or naval unit without permission.
- (of a place) Abandoned, without people.
- Desolate