The most common and most likely answer to this crossword clue is the 4 plus 2 plus 5 plus 4 letter word REST ON ONE’S OARS
I've seen this crossword clue in Financial Times
R
E
S
T
O
N
O
N
E
â
€
™
S
O
A
R
S
Buddy explains!
The straight part
We know that "Calm down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case calm down = rest on one’s oars.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.