The most common and most likely answer to this crossword clue is the 18 letter word SPUR-OF-THE-MOMENT
I've seen this crossword clue in Financial Times
S
P
U
R
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O
F
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T
H
E
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M
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N
T
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.
What does SPUR-OF-THE-MOMENT mean?
SPUR-OF-THE-MOMENT adjective- Spontaneous; impromptu.