The most common and most likely answer to this crossword clue is the 10 letter word CONSONANTS
I've seen this crossword clue in The Daily Telegraph
C
O
N
S
O
N
A
N
T
S
Buddy explains!

The straight part
We know that "Letters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case letters = consonants.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 CONSONANTS mean?
CONSONANTS noun- A sound that results from the passage of air through restrictions of the oral cavity; any sound that is not the dominant sound of a syllable, the dominant sound generally being a vowel.
- A letter representing the sound of a consonant.