The most common and most likely answer to this crossword clue is the 10 letter word PHLEGMATIC
I've seen this crossword clue in The Guardian
P
H
L
E
G
M
A
T
I
C
More clues leading to the result PHLEGMATIC
Buddy explains!

The straight part
We know that "of calm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of calm = phlegmatic.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 PHLEGMATIC mean?
PHLEGMATIC noun- One who has a phlegmatic disposition.
- Not easily excited to action or passion; calm; sluggish.
- Abounding in phlegm.
- Generating, causing, or full of phlegm.