The most common and most likely answer to this crossword clue is the 8 plus 10 letter word EIGHTEEN FIFTY-FIVE
I've seen this crossword clue in The Guardian
E
I
G
H
T
E
E
N
F
I
F
T
Y
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F
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V
E
Buddy explains!
The straight part
We know that "Nearly seven/pillar boxes then appeared in London" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nearly seven/pillar boxes then appeared in london = eighteen fifty-five.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.