The most common and most likely answer to this crossword clue is the 11 letter word UNFLAPPABLE
I've seen this crossword clue in The Daily Telegraph
U
N
F
L
A
P
P
A
B
L
E
Buddy explains!

The straight part
We know that "Calm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case calm = unflappable.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.
Crossword Answer definitions
UNFLAPPABLE adjective- Remaining composed and level-headed at all times; impossible to fluster; not becoming frustrated or irritated easily.