The most common and most likely answer to this crossword clue is the 8 letter word CREOSOTE
I've seen this crossword clue in The Guardian
C
R
E
O
S
O
T
E
More clues leading to the result CREOSOTE
- Scooter treated with European wood preservative
- Oily liquid ruining eco-store
- Dark liquid drunk by Osage leader, native American bottles
- Wood preservative, very big over in Greek island
- In endless crawl, old wino consumed oily liquid
- What island stocks is old, therefore preservative is the answer
- It preserves energy and oxygen when getting into tight corset
- Legendary bull’s home pumped with oxygen thus, as preservative
- Black liquid, supplied by Native American, old drunk swallowed
- Island holds nothing so useful for preservation
Buddy explains!

The straight part
We know that "preservative" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case preservative = creosote.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 CREOSOTE mean?
CREOSOTE noun- A pale yellow oily liquid, containing phenols and similar compounds, obtained by the destructive distillation of wood tar, once used medicinally.
- A similar brown liquid obtained from coal tar used as a wood preservative.
- The creosote bush.
- To apply creosote.