The most common and most likely answer to this crossword clue is the 7 letter word IGNITES
I
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The straight part
We know that "Set fire to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case set fire to = ignites.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 IGNITES mean?
IGNITES verb- To set fire to (something), to light (something)
- To spark off (something), to trigger
- To commence burning.
- To subject to the action of intense heat; to heat strongly; often said of incombustible or infusible substances.