The most common and most likely answer to this crossword clue is the 5 plus 5 letter word BLIND ALLEY
I've seen this crossword clue in The Guardian
B
L
I
N
D
A
L
L
E
Y
More clues leading to the result BLIND ALLEY
Buddy explains!

The straight part
We know that "cul-de-sac" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cul-de-sac = blind alley.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.