The most common and most likely answer to this crossword clue is the 8 letter word ABSEILER
I've seen this crossword clue in The Guardian
A
B
S
E
I
L
E
R
More clues leading to the result ABSEILER
- Sailor upset banks: he’s let himself down?
- Mariner heard behind another person making descent
- Person climbing down depends on people flying up
- Boatmansaid to be a descender?
- Sailor relies sadly on one fast going down
- Seaman, then another, reported one clinging to cliff-face?
- Rock hanger-on, getting excited, is able to meet Queen
Buddy explains!
The straight part
We know that "one fast going down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one fast going down = abseiler.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.