The most common and most likely answer to this crossword clue is the 4 plus 2 plus 1 plus 8 letter word COOL AS A CUCUMBER
C
O
O
L
A
S
A
C
U
C
U
M
B
E
R
Buddy explains!
The straight part
We know that "Composed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case composed = cool as a cucumber.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.