The most common and most likely answer to this crossword clue is the 6 letter word PHLEGM
I've seen this crossword clue in The Times
P
H
L
E
G
M
More clues leading to the result PHLEGM
- Calm senior politician enlisting hard supporter
- Being collected from public house, left meeting
- Calmness of posh little gym, all empty
- Help out gathering material primarily used for humour
- Cool Belgian briefly spoken?
- Basic measurement on mass tendency not to react
- Help out with game, oddly showing self-possession
- Hospital support limited by premier’s indifference
- Brought up in front of the meeting a scale model finally
- Level of acidity on Malta that’s brought up during cold spell
Buddy explains!

The straight part
We know that "Calmness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case calmness = phlegm.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 PHLEGM mean?
PHLEGM noun- One of the four humors making up the body in ancient and mediaeval medicine; said to be cold and moist, and often identified with mucus.
- Viscid mucus produced by the body, later especially mucus expelled from the bronchial passages by coughing.
- A watery distillation, especially one obtained from plant matter; an aqueous solution.
- Calmness of temperament, composure; also seen negatively, sluggishness, indifference.