The most common and most likely answer to this crossword clue is the 7 plus 3 letter word LINSEED OIL
I've seen this crossword clue in The Guardian
L
I
N
S
E
E
D
O
I
L
More clues leading to the result LINSEED OIL
- Sell iodine compound that keeps bats in good nick
- Oldie’s line upset for natural preservative
- Sell iodine prepared as a preservative
- Sell iodine, perhaps, as a substance in paint
- Sell iodine compound for application in sport
- Careless orderlies leave luxury car after cutting short the tie-up for a paint ingredient
Buddy explains!
The straight part
We know that "natural preservative" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case natural preservative = linseed oil.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.