The most common and most likely answer to this crossword clue is the 6 plus 4 letter word MARBLE ARCH
I've seen this crossword clue in The Times
M
A
R
B
L
E
A
R
C
H
More clues leading to the result MARBLE ARCH
- Spoil with cleaning fluid exterior to Queen’s London landmark
- London landmark, indistinct during parade
- Labour politician nearly arrested by protest in London landmark
- Half-hearted rabble disorderly in protest in part of London
- Demo going round obscure London landmark
- Faint during demonstration somewhere in London
Buddy explains!
The straight part
We know that "London landmark" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case london landmark = marble arch.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.