The most common and most likely answer to this crossword clue is the 10 letter word CONTRARIAN
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The straight part
We know that "Rejecting popular opinion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rejecting popular opinion = contrarian.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 CONTRARIAN mean?
CONTRARIAN noun- A person who likes or tends to express a contradicting viewpoint, especially one who denounces the majority persuasion.
- A financial investor who tends to have an opinion of market trends at variance with most others.