The most common and most likely answer to this crossword clue is the 8 letter word SCUDDING
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Buddy explains!
The straight part
We know that "moving very fast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case moving very fast = scudding.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 SCUDDING mean?
SCUDDING verb- To race along swiftly (especially used of clouds).
- To run, or be driven, before a high wind with no sails set.
- To hit or slap.
- To speed.
- To skim flat stones so they skip along the water.
- The action of the verb to scud.
- That scuds or scud.