The most common and most likely answer to this crossword clue is the 6 letter word DRONES
I've seen this crossword clue in The Guardian
D
R
O
N
E
S
More clues leading to the result DRONES
Buddy explains!

The straight part
We know that "states monotonously" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case states monotonously = drones.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 DRONES mean?
DRONES noun- A male ant, bee or wasp, which does not work but can fertilize the queen bee.
- Someone who does not work; a lazy person, an idler.
- One who performs menial or tedious work.
- A remotely controlled aircraft, an unmanned aerial vehicle (UAV).
- A low-pitched hum or buzz.
- One of the fixed-pitch pipes on a bagpipe.
- A genre of music that uses repeated lengthy droning sounds.
- A humming or deep murmuring sound.