The most common and most likely answer to this crossword clue is the 9 letter word BILLIONTH
I've seen this crossword clue in The Guardian
B
I
L
L
I
O
N
T
H
More clues leading to the result BILLIONTH
Buddy explains!
The straight part
We know that "Fraction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fraction = billionth.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 BILLIONTH mean?
BILLIONTH noun- The person or thing in the billionth position.
- One of a billion equal parts of a whole.
- The ordinal form of the number one billion.