The most common and most likely answer to this crossword clue is the 8 letter word GRIEVING
I've seen this crossword clue in The Daily Telegraph
G
R
I
E
V
I
N
G
More clues leading to the result GRIEVING
Buddy explains!

The straight part
We know that "in mourning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in mourning = grieving.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 GRIEVING mean?
GRIEVING verb- To cause sorrow or distress to.
- To feel very sad about; to mourn; to sorrow for.
- To experience grief.
- To harm.
- To submit or file a grievance (about).
- An act or instance of feeling or expressing grief.