The most common and most likely answer to this crossword clue is the 9 letter word ALPHABETS
A
L
P
H
A
B
E
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 = alphabets.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 ALPHABETS mean?
ALPHABETS noun- The set of letters used when writing in a language.
- A writing system in which letters represent phonemes. (Contrast e.g. logography, a writing system in which each character represents a word, and syllabary, in which each character represents a syllable.)
- A typically finite set of distinguishable symbols.
- An individual letter of an alphabet; an alphabetic character.
- The simplest rudiments; elements.
- To designate by the letters of the alphabet; to arrange alphabetically.