The most common and most likely answer to this crossword clue is the 8 letter word FLUORIDE
F
L
U
O
R
I
D
E
Buddy explains!
The straight part
We know that "Salt in your toothpaste?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case salt in your toothpaste? = fluoride.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 FLUORIDE mean?
FLUORIDE noun- Any salt of hydrofluoric acid; for example, potassium fluoride.
- A binary compound of fluorine and another element or radical.