The most common and most likely answer to this crossword clue is the 9 letter word HYDRAZINE
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The straight part
We know that "This can be used as rocket fuel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this can be used as rocket fuel = hydrazine.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 HYDRAZINE mean?
HYDRAZINE noun- A corrosive, fuming liquid, NH2-NH2, used as a rocket fuel.
- Any member of the class of organic compounds formally derived from NH2-NH2.