The most common and most likely answer to this crossword clue is the 16 letter word WILL-O’-THE-WISP
I've seen this crossword clue in The Guardian
W
I
L
L
-
O
â
€
™
-
T
H
E
-
W
I
S
P
Buddy explains!

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.