The most common and most likely answer to this crossword clue is the 4 plus 8 letter word FAIT ACCOMPLI
I've seen this crossword clue in The Times
F
A
I
T
A
C
C
O
M
P
L
I
Buddy explains!
The straight part
We know that "At the Sorbonne, it’s the done thing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case at the sorbonne, it’s the done thing = fait accompli.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.