The most common and most likely answer to this crossword clue is the 6 letter word GLOOMY
I've seen this crossword clue in The Guardian
G
L
O
O
M
Y
More clues leading to the result GLOOMY
- Poorly-lit weaving machine blocking outside of gallery
- Dark, unopened icy house, extremely musty
- Dull, ultimately depressing as a cotton mill?
- Number one from George, John and Paul’s dismal
- Head from frozen residence with heavens overcast
- Setter leaving cold building with masonry walls is blue!
- Despondent individual leaving White House with expression of surprise
- Dark logo to be changed – gracious me!
- Shadowy overhang built in to gallery’s wings
- Last of oil covered in sticky substance, well black
Buddy explains!

The straight part
We know that "dismal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dismal = gloomy.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 GLOOMY mean?
GLOOMY adjective- Not very illuminated; dim because of darkness, especially when appearing depressing or frightening.
- Suffering from gloom; melancholy; dejected.