The most common and most likely answer to this crossword clue is the 9 letter word PROPRIETY
I've seen this crossword clue in The Times
P
R
O
P
R
I
E
T
Y
More clues leading to the result PROPRIETY
Buddy explains!
The straight part
We know that "Decorum" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decorum = propriety.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 PROPRIETY mean?
PROPRIETY noun- The particular character or essence of someone or something; individuality.
- A characteristic; an attribute.
- A piece of land owned by someone; someone's property.
- More generally, something owned by someone; a possession.
- The fact of possessing something; ownership.
- Correct language or pronunciation.
- Suitability, fitness; the quality of being appropriate.
- (often in the plural) Correctness in behaviour and morals; good manners, seemliness.