The most common and most likely answer to this crossword clue is the 8 letter word RAPIDEST
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More clues leading to the result RAPIDEST
Buddy explains!

The straight part
We know that "Fastest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fastest = rapidest.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 RAPIDEST mean?
RAPIDEST adjective- Very swift or quick.
- Steep, changing altitude quickly. (of a slope)
- Needing only a brief exposure time. (of a lens, plate, film, etc.)
- Violent, severe.
- Happy.