The most common and most likely answer to this crossword clue is the 9 letter word ECTOPLASM
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "in which the spirit leaks out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in which the spirit leaks out = ectoplasm.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 ECTOPLASM mean?
ECTOPLASM noun- A visible substance believed to emanate from the body of a spiritualistic medium during communication with the dead.
- An immaterial or ethereal substance, especially the transparent corporeal presence of a spirit or ghost.
- The outer granule-free layer of cytoplasm.