The most common and most likely answer to this crossword clue is the 5 plus 6 letter word TOWER BRIDGE
I've seen this crossword clue in Financial Times
T
O
W
E
R
B
R
I
D
G
E
More clues leading to the result TOWER BRIDGE
- Girder to be fixed outside with adjustable spanner
- Drag performer tailed Irish girl in London landmark
- Adjustable borer and widget for adjustable spanner?
- Wrote about game, a fixture between two Thames boroughs
- Wrote about game, a fixture between two London boroughs
- Modify widget with borer and adjustable spanner
Buddy explains!
The straight part
We know that "a fixture between two London boroughs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a fixture between two london boroughs = tower bridge.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.