The most common and most likely answer to this crossword clue is the 6 letter word STARCH
I've seen this crossword clue in The Times
S
T
A
R
C
H
More clues leading to the result STARCH
- Virtuous person knowing formal behaviour
- Formality of celebrity and Companion of Honour
- Holy person with cunning formality
- Glamorous person at head of church shows formality
- Celebrity on church formality
- Stiffener for fabric of structure to right of street
- Before presentation of award, celebrity needs stiffener
- Celebrity cheerleaders’ energy store
- Stiff found in church on street near the centre of Kilmarnock
- *carbon and hydrogen – carbohydrate!
Buddy explains!

The straight part
We know that "formal behaviour" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case formal behaviour = starch.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 STARCH mean?
STARCH noun- A widely diffused vegetable substance found especially in seeds, bulbs, and tubers, and extracted (as from potatoes, corn, rice, etc.) as a white, glistening, granular or powdery substance, without taste or smell, and giving a very peculiar creaking sound when rubbed between the fingers. It is used as a food, in the production of commercial grape sugar, for stiffening linen in laundries, in making paste, etc.
- (nutrition) Carbohydrates, as with grain and potato based foods.
- A stiff, formal manner; formality.
- Any of various starch-like substances used as a laundry stiffener
- To apply or treat with laundry starch, to create a hard, smooth surface.
- Stiff; precise; rigid.