The most common and most likely answer to this crossword clue is the 15 letter word WILL-O-THE-WISP
I've seen this crossword clue in Financial Times
W
I
L
L
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O
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T
H
E
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W
I
S
P
More clues leading to the result WILL-O-THE-WISP
Buddy explains!
The straight part
We know that "Ghostly light" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ghostly light = will-o-the-wisp.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.