The most common and most likely answer to this crossword clue is the 5 letter word FLITS
I've seen this crossword clue in Financial Times
F
L
I
T
S
Buddy explains!
The straight part
We know that "moves quickly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case moves quickly = flits.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 FLITS mean?
FLITS noun- A fluttering or darting movement.
- A particular, unexpected, short lived change of state.
- A homosexual.
- To move about rapidly and nimbly.
- To move quickly from one location to another.
- To unpredictably change state for short periods of time.
- To move house (sometimes a sudden move to avoid debts).
- To be unstable; to be easily or often moved.