The most common and most likely answer to this crossword clue is the 4 plus 5 plus 5 plus 2 letter word KEEP ONE’S SHIRT ON
I've seen this crossword clue in The Guardian
K
E
E
P
O
N
E
â
€
™
S
S
H
I
R
T
O
N
More clues leading to the result KEEP ONE’S SHIRT ON
Buddy explains!
The straight part
We know that "Stay calm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stay calm = keep one’s shirt on.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.