The most common and most likely answer to this crossword clue is the 6 letter word SADDEN
I've seen this crossword clue in Financial Times
S
A
D
D
E
N
More clues leading to the result SADDEN
Buddy explains!

The straight part
We know that "to get one down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to get one down = sadden.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 SADDEN mean?
SADDEN verb- To make sad or unhappy.
- To become sad or unhappy.
- To darken a color during dyeing.
- To render heavy or cohesive.