The most common and most likely answer to this crossword clue is the 4 letter word QUIN
I've seen this crossword clue in Financial Times
Q
U
I
N
Buddy explains!

The straight part
We know that "One in five" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in five = quin.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 QUIN mean?
QUIN noun- A quintuplet.
- A European scallop, Pecten opercularis, used as food.