The most common and most likely answer to this crossword clue is the 9 letter word BREAKNECK
I've seen this crossword clue in The Guardian
B
R
E
A
K
N
E
C
K
More clues leading to the result BREAKNECK
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 = breakneck.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 BREAKNECK mean?
BREAKNECK noun- A fall that breaks the neck.
- A dangerous steep place from which one could fall.
- Dangerously fast; hell-for-leather.