The most common and most likely answer to this crossword clue is the 11 letter word SELF-ESTEEM
I've seen this crossword clue in Financial Times
S
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More clues leading to the result SELF-ESTEEM
Buddy explains!
The straight part
We know that "Conviction that number one is positive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case conviction that number one is positive = self-esteem.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 SELF-ESTEEM mean?
SELF-ESTEEM noun- Confidence in one's own worth; self-respect.