The most common and most likely answer to this crossword clue is the 6 letter word SPEEDY
I've seen this crossword clue in Financial Times
S
P
E
E
D
Y
More clues leading to the result SPEEDY
- Swift, see, crossing river to the west
- Fast, agent crossing river to the west
- Regularly defends having clock outside running fast
- Fast river rising – see to contain it
- Over ebbing river, spot fleet
- Fast agent crossing river to the west
- Quick to become unwell carrying priest
- Mangy bears start to perish fast
- Soon achieved in property originally having shabby exterior
- Even characters in Aeneid going aboard make out fleet
Buddy explains!

The straight part
We know that "fleet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fleet = speedy.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 SPEEDY mean?
SPEEDY verb- To process in a faster than normal, accelerated way
- (Wiktionary and WMF jargon) to apply the speedy rule in an online community (often the deletion rule); speedy delete
- Rapid; swift