The most common and most likely answer to this crossword clue is the 9 letter word PHOSPHATE
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The straight part
We know that "Salt" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case salt = phosphate.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 PHOSPHATE mean?
PHOSPHATE noun- Any salt or ester of phosphoric acid.
- A carbonated soft drink sweetened with fruit syrup and with some phosphoric acid.
- To treat or coat with a phosphate or with phosphoric acid