The most common and most likely answer to this crossword clue is the 6 letter word OMEGAS
I've seen this crossword clue in The Guardian
O
M
E
G
A
S
Buddy explains!
The straight part
We know that "Last letters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case last letters = omegas.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 OMEGAS mean?
OMEGAS noun- The twenty-fourth letter of the Classical and the Modern Greek alphabet, and the twenty-eighth letter of the Old and the Ancient Greek alphabet, i.e. the last letter of every Greek alphabet. Uppercase version: Ω; lowercase: ω.
- (often capitalized) The end; the final, last or ultimate in a sequence.
- Angular velocity; symbol: ω.
- An omega male.
- The percentage change in an option value divided by the percentage change in the underlying asset's price.
- In omegaverse fiction, a person of a sexually-submissive (and sometimes secondary) gender/sex that is driven by biology, magic, or other means to bond with an alpha, with males of this type often being able to get pregnant.