Cryptic crossword clue
Fuel additive in tank, I conk out 9

The most common and most likely answer to this crossword clue is the 9 letter word ANTIKNOCK

I've seen this crossword clue in The Daily Telegraph
A
N
T
I
K
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O
C
K

Buddy explains!

The straight part
We know that "Fuel additive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fuel additive = antiknock.
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 ANTIKNOCK mean?

ANTIKNOCK noun
  1. Such a substance, usually tetraethyl lead (also MMT, MTBE and ethanol)
ANTIKNOCK adjective
  1. Of an additive added to petroleum/gasoline to reduce the occurrence of engine knock.

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