The most common and most likely answer to this crossword clue is the 7 letter word TOLUENE
I've seen this crossword clue in The Times
T
O
L
U
E
N
E
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The straight part
We know that "Flammable liquid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flammable liquid = toluene.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 TOLUENE mean?
TOLUENE noun- A colourless, inflammable liquid hydrocarbon, methylbenzene, CH3.C6H5, used as a solvent, in high-octane fuels and in the production of many chemical compounds.