The most common and most likely answer to this crossword clue is the 12 letter word DISCOURAGING
I've seen this crossword clue in The Times
D
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S
C
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A
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Buddy explains!

The straight part
We know that "depressing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case depressing = discouraging.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 DISCOURAGING mean?
DISCOURAGING verb- To extinguish the courage of; to dishearten; to depress the spirits of; to deprive of confidence; to deject.
- To persuade somebody not to do (something).
- Discouragement
- That causes discouragement