The most common and most likely answer to this crossword clue is the 15 letter word NINETEEN-THIRTY
I've seen this crossword clue in The Guardian
N
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N
E
T
E
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N
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T
H
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T
Y
Buddy explains!
The straight part
We know that "Half past seven in the evening/a time of depression" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case half past seven in the evening/a time of depression = nineteen-thirty.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.