The most common and most likely answer to this crossword clue is the 10 letter word CUL-DE-SAC
I've seen this crossword clue in The Times
C
U
L
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D
E
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S
A
C
Buddy explains!

The straight part
We know that "Downing Street perhaps" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case downing street perhaps = cul-de-sac.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.
What does CUL-DE-SAC mean?
CUL-DE-SAC noun- A blind alley or dead end street.
- A circular area at the end of a dead end street to allow cars to turn around, designed so children can play on the street, with little or no through-traffic.
- An impasse.
- A sack-like cavity, a tube open at one end only.