The most common and most likely answer to this crossword clue is the 13 letter word IMPERTURBABLE
I've seen this crossword clue in The Daily Telegraph
I
M
P
E
R
T
U
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B
A
B
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More clues leading to the result IMPERTURBABLE
Buddy explains!

The straight part
We know that "Untroubled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case untroubled = imperturbable.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 IMPERTURBABLE mean?
IMPERTURBABLE adjective- Not easily perturbed, upset or excited.
- Calm and collected, even under pressure.