The most common and most likely answer to this crossword clue is the 7 letter word RESTFUL
I've seen this crossword clue in Financial Times
R
E
S
T
F
U
L
More clues leading to the result RESTFUL
- At peace, basking in purest fulfilment
- Calm displayed by Bucharest fullbacks
- Tranquil end to summer with flutes playing
- Fluster-free?
- Calm those remaining taking on oil, say, when energy’s lost
- Agitate, fluster, calm
- Still in a terrible fluster
- Relaxing, fluster having been resolved
- In a flap, fluster, needing to get calm
- Calm end to summer with flutes playing
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 = restful.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 RESTFUL mean?
RESTFUL adjective- Using REST (representational state transfer).
- Peaceful; having a peaceful aspect
- Restorative; aiding rest