The most common and most likely answer to this crossword clue is the 10 letter word COLOURLESS
I've seen this crossword clue in The Times
C
O
L
O
U
R
L
E
S
S
More clues leading to the result COLOURLESS
Buddy explains!

The straight part
We know that "Dreary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dreary = colourless.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 COLOURLESS mean?
COLOURLESS adjective- Having little or no colour.
- (of a liquid) Water white.
- Lacking in interest or variety.