The most common and most likely answer to this crossword clue is the 8 letter word DRY-CURE
I've seen this crossword clue in The Guardian
D
R
Y
-
C
U
R
E
Buddy explains!

The straight part
We know that "preserve by adding salt" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case preserve by adding salt = dry-cure.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 DRY-CURE mean?
DRY-CURE verb- To preserve food using salt (and sometimes sodium nitrite)