The most common and most likely answer to this crossword clue is the 12 letter word SUPERSTITION
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result SUPERSTITION
Buddy explains!

The straight part
We know that "irrational belief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case irrational belief = superstition.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 SUPERSTITION mean?
SUPERSTITION noun- A belief or beliefs, not based on human reason or scientific knowledge, that events may be influenced by one's behaviour in some magical or mystical way.
- Excessive nicety; overscrupulousness.