The most common and most likely answer to this crossword clue is the 4 plus 3 plus 3 letter word SINE QUA NON
I've seen this crossword clue in The Guardian
S
I
N
E
Q
U
A
N
O
N
Buddy explains!
The straight part
We know that "An essential for the Latinist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an essential for the latinist = sine qua non.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.