The most common and most likely answer to this crossword clue is the 10 letter word CONVICTION
I've seen this crossword clue in The Guardian
C
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V
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More clues leading to the result CONVICTION
Buddy explains!

The straight part
We know that "Firm belief/Firm belief/guilty verdict /guilty verdict" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case firm belief/firm belief/guilty verdict /guilty verdict = conviction.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 CONVICTION mean?
CONVICTION noun- A firmly held belief.
- A judgement of guilt in a court of law.
- The state of being found or proved guilty.
- The state of being wholly convinced.