The most common and most likely answer to this crossword clue is the 9 letter word INTUITIVE
I've seen this crossword clue in The Daily Telegraph
I
N
T
U
I
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V
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More clues leading to the result INTUITIVE
Buddy explains!
The straight part
We know that "There’s no reason to be so/spontaneous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case there’s no reason to be so/spontaneous = intuitive.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 INTUITIVE mean?
INTUITIVE noun- One who has (especially parapsychological) intuition.
- Spontaneous, without requiring conscious thought.
- Easily understood or grasped by intuition.
- Having a marked degree of intuition.