The most common and most likely answer to this crossword clue is the 4 plus 7 letter word BIEN ENTENDU
I've seen this crossword clue in The Times
B
I
E
N
E
N
T
E
N
D
U
More clues leading to the result BIEN ENTENDU
Buddy explains!

The straight part
We know that "Of course" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of course = bien entendu.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.