The most common and most likely answer to this crossword clue is the 14 letter word FUNDAMENTALISM
I've seen this crossword clue in The Times
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More clues leading to the result FUNDAMENTALISM
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The straight part
We know that "orthodoxy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case orthodoxy = fundamentalism.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 FUNDAMENTALISM mean?
FUNDAMENTALISM noun- The tendency to reduce a religion to its most fundamental tenets, based on strict interpretation of core texts.
- (by extension) A rigid conformity to any set of basic tenets.
- The belief that fundamental financial quantities are the best predictor of the price of a financial instrument.