The most common and most likely answer to this crossword clue is the 6 plus 8 letter word DOUBLE ENTENDRE
I've seen this crossword clue in The Daily Telegraph
D
O
U
B
L
E
E
N
T
E
N
D
R
E
More clues leading to the result DOUBLE ENTENDRE
Buddy explains!
The straight part
We know that "French sauce delivered inadvertently?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case french sauce delivered inadvertently? = double entendre.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.