The most common and most likely answer to this crossword clue is the 10 letter word EVACUATION
I've seen this crossword clue in The Guardian
E
V
A
C
U
A
T
I
O
N
More clues leading to the result EVACUATION
Buddy explains!

The straight part
We know that "Leaving/1" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leaving/1 = evacuation.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 EVACUATION mean?
EVACUATION noun- The act of evacuating; leaving a place in an orderly fashion, especially for safety.
- Withdrawal of troops or civils from a town, fortress, etc.
- The act of emptying, clearing of the contents, or discharging, including creating a vacuum.
- Voidance of any matter by the natural passages of the body or by an artificial opening; defecation; also, a diminution of the fluids of an animal body by cathartics, venesection, or other means.
- That which is evacuated or discharged; especially, a discharge by stool or other natural means.
- Abolition; nullification.