The most common and most likely answer to this crossword clue is the 10 letter word PROFESSION
I've seen this crossword clue in The Guardian
P
R
O
F
E
S
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Buddy explains!

The straight part
We know that "Calling/declaration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case calling/declaration = profession.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 PROFESSION mean?
PROFESSION noun- A declaration of belief, faith or one's opinion, whether genuine or pretended.
- An occupation, trade, craft, or activity in which one has a professed expertise in a particular area; a job, especially one requiring a high level of skill or training.
- The practitioners of such an occupation collectively.
- A promise or vow made on entering a religious order.