Cryptic crossword clue
Heart of Mine: revolutionary’s Obviously 5 Believers actually none

The most common and most likely answer to this crossword clue is the 8 letter word INFIDELS

I've seen this crossword clue in Independent
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Buddy explains!

The straight part
We know that "Believers (actually none)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case believers (actually none) = infidels.
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 INFIDELS mean?

INFIDELS noun
  1. (now usually derogatory) One who does not believe in a certain religion.
  2. (now usually derogatory) One who does not believe in a certain principle.
  3. (now usually derogatory) One with no religious beliefs.

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