The most common and most likely answer to this crossword clue is the 5 plus 8 letter word PRIME MERIDIAN
I've seen this crossword clue in The Daily Telegraph
P
R
I
M
E
M
E
R
I
D
I
A
N
Buddy explains!
The straight part
We know that "line through London" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case line through london = prime meridian.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.