The most common and most likely answer to this crossword clue is the 7 letter word AMYLASE
I've seen this crossword clue in The Guardian
A
M
Y
L
A
S
E
Buddy explains!
The straight part
We know that "What’s in saliva to break up meal, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’s in saliva to break up meal, say = amylase.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 AMYLASE mean?
AMYLASE noun- Any of a class of digestive enzymes, present in saliva, that break down complex carbohydrates such as starch into simpler sugars such as glucose.