The most common and most likely answer to this crossword clue is the 3 letter word SUR
S
U
R
More clues leading to the result SUR
Buddy explains!
The straight part
We know that "What’s on in Paris/It’s big in California" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’s on in paris/it’s big in california = sur.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.