The most common and most likely answer to this crossword clue is the 13 letter word REPUBLICANISM
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More clues leading to the result REPUBLICANISM
Buddy explains!
The straight part
We know that "What some politicians believe in" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what some politicians believe in = republicanism.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 REPUBLICANISM mean?
REPUBLICANISM noun- The political ideology of being a citizen in a state as a republic under which the people hold popular sovereignty.