The most common and most likely answer to this crossword clue is the 10 letter word FAITHFULLY
I've seen this crossword clue in The Times
F
A
I
T
H
F
U
L
L
Y
Buddy explains!

The straight part
We know that "In composing letter, I may follow yours/without variation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in composing letter, i may follow yours/without variation = faithfully.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 FAITHFULLY mean?
FAITHFULLY adverb- In a faithful manner.
- A conventional formula for ending a letter, used when the salutation addresses the person for whom the letter is intended using an honorific.