The most common and most likely answer to this crossword clue is the 3 plus 2 plus 4 letter word ONE IN FIVE
I've seen this crossword clue in Financial Times
O
N
E
I
N
F
I
V
E
Buddy explains!
The straight part
We know that "May be Julian, if famous/20%" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case may be julian, if famous/20% = one in five.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.