The most common and most likely answer to this crossword clue is the 10 letter word HYPERSONIC
I've seen this crossword clue in The Daily Telegraph
H
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More clues leading to the result HYPERSONIC
Buddy explains!
The straight part
We know that "very fast?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very fast? = hypersonic.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 HYPERSONIC mean?
HYPERSONIC adjective- (of a speed) equal to, or greater than, or capable of achieving, five times the speed of sound.
- (of a speed) far enough above the speed of sound as to cause significant differences in behaviour due to chemical reactions or disassociation of the air.