The most common and most likely answer to this crossword clue is the 7 letter word PERCENT
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More clues leading to the result PERCENT
Buddy explains!

The straight part
We know that "one in hundred" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in hundred = percent.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 PERCENT mean?
PERCENT noun- A percentage, a proportion (especially per hundred).
- One part per hundred; one percent.
- For every hundred (used with preceding numeral to form a noun phrase expressing a proportion).