The most common and most likely answer to this crossword clue is the 10 letter word DEPRESSANT
I've seen this crossword clue in The Guardian
D
E
P
R
E
S
S
A
N
T
Buddy explains!

The straight part
We know that "downer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case downer = depressant.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 DEPRESSANT mean?
DEPRESSANT noun- A pharmacological substance which decreases neuronal or physiological activity.
- An agent that inhibits the flotation of a mineral or minerals.
- (pharmaceutical effect) reducing functional or nervous activity.