The most common and most likely answer to this crossword clue is the 16 letter word HACKNEYÂ CARRIAGE
I've seen this crossword clue in The Guardian
H
A
C
K
N
E
Y
Â
C
A
R
R
I
A
G
E
Buddy explains!
The straight part
We know that "London Taxi" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case london taxi = hackney carriage.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.