The most common and most likely answer to this crossword clue is the 4 plus 4 letter word LEAD FREE
I've seen this crossword clue in Independent
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More clues leading to the result LEAD FREE
Buddy explains!
The straight part
We know that "this type of fuel?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this type of fuel? = lead free.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 LEAD FREE mean?
LEAD FREE adjective- Containing no lead.
- (of petroleum) Containing no tetraethyl lead; unleaded.