The most common and most likely answer to this crossword clue is the 11 letter word CARBURETTOR
I've seen this crossword clue in The Guardian
C
A
R
B
U
R
E
T
T
O
R
Buddy explains!
The straight part
We know that "Air and fuel are mixed in this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case air and fuel are mixed in this = carburettor.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 CARBURETTOR mean?
CARBURETTOR noun- A device in an internal combustion engine where fuel is vaporized and mixed with air prior to ignition.
- A water pipe or bong; a device or contrivance for mixing air with burning cannabis or cocaine.