The most common and most likely answer to this crossword clue is the 10 letter word UNIFORMITY
I've seen this crossword clue in Financial Times
U
N
I
F
O
R
M
I
T
Y
More clues leading to the result UNIFORMITY
- Further education class on computer work — why it’s monotonous
- Class one joins in harmony becomes monotony
- In one class where everything is the same
- One capturing potentially first class likeness
- Standard behaviour for me, participating in agreement?
- Ultimately you auction Dali off to our museum — Saatchi not happy with lack of diversity
Buddy explains!

The straight part
We know that "monotony" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monotony = uniformity.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 UNIFORMITY mean?
UNIFORMITY noun- The state of being uniform, alike and lacking variety.
- The absence of alternatives or diversity; sameness.