The most common and most likely answer to this crossword clue is the 5 letter word RAPID
I've seen this crossword clue in The Times
R
A
P
I
D
More clues leading to the result RAPID
- Fleet's pressure in attack
- Fleet's power in attack
- Very fast unexpected attack grabbing power
- Swift drop, a run in the wrong direction
- Fleet inspector retires following censure
- Swift to criticise papers
- Swift attack seizing power
- Quick to condemn primitive instincts
- Quick attack trapping leader of pack
- Fast hit I had wanted
Buddy explains!

The straight part
We know that "Fast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fast = rapid.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 RAPID mean?
RAPID noun- (often in the plural) a rough section of a river or stream which is difficult to navigate due to the swift and turbulent motion of the water.
- A burst of rapid fire.
- 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.
- (archaic or colloquial) Rapidly.