The most common and most likely answer to this crossword clue is the 18 letter word SPUR-OF-THE-MOMENT
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Buddy explains!
The straight part
We know that "Impetuous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case impetuous = spur-of-the-moment.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.