The most common and most likely answer to this crossword clue is the 7 letter word STILLED
I've seen this crossword clue in The Times
S
T
I
L
L
E
D
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 = stilled.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 STILLED mean?
STILLED verb- To calm down, to quiet
- To trickle, drip.
- To cause to fall by drops.
- To expel spirit from by heat, or to evaporate and condense in a refrigeratory; to distill.
- Brought to a standstill
- Quieted