The most common and most likely answer to this crossword clue is the 7 letter word COCKNEY
I've seen this crossword clue in The Daily Telegraph
C
O
C
K
N
E
Y
More clues leading to the result COCKNEY
Buddy explains!

The straight part
We know that "Londoner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case londoner = cockney.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 COCKNEY mean?
COCKNEY noun- A native or inhabitant of parts of the East End of London
- The accent and speech mannerisms of these people
- An effeminate person; a spoilt child.
- Of, or relating to these people or their accent