The most common and most likely answer to this crossword clue is the 9 letter word EXPRESSES
I've seen this crossword clue in The Guardian
E
X
P
R
E
S
S
E
S
Buddy explains!
The straight part
We know that "Clearly states/they’re fast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clearly states/they’re fast = expresses.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 EXPRESSES mean?
EXPRESSES noun- A mode of transportation, often a train, that travels quickly or directly.
- A service that allows mail or money to be sent rapidly from one destination to another.
- An express rifle.
- A clear image or representation; an expression; a plain declaration.
- A messenger sent on a special errand; a courier.
- An express office.
- That which is sent by an express messenger or message.
- The action of conveying some idea using words or actions; communication, expression.
- A specific statement or instruction.
- To convey or communicate; to make known or explicit.
- To press, squeeze out (especially said of milk).
- To translate messenger RNA into protein.
- To transcribe deoxyribonucleic acid into messenger RNA.