The most common and most likely answer to this crossword clue is the 7 letter word UNFAZED
I've seen this crossword clue in The Guardian
U
N
F
A
Z
E
D
Buddy explains!

The straight part
We know that "remaining calm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case remaining calm = unfazed.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 UNFAZED mean?
UNFAZED adjective- Not frightened or hesitant; undaunted; not put off.
- Undamaged