The most common and most likely answer to this crossword clue is the 10 letter word PHILATELIC
I've seen this crossword clue in The Times
P
H
I
L
A
T
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L
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C
More clues leading to the result PHILATELIC
Buddy explains!
The straight part
We know that "related to something on French letter?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case related to something on french letter? = philatelic.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.