Cryptic crossword clue
Fuel ingredient at once unstable 6

The most common and most likely answer to this crossword clue is the 6 letter word OCTANE

I've seen this crossword clue in Financial Times
O
C
T
A
N
E

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

OCTANE noun
  1. Any of the eighteen isomeric aliphatic hydrocarbons (C8H18) found in petroleum, especially an iso-octane 2,2,4 trimethyl-pentane; they are used as fuels and solvents.

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