The most common and most likely answer to this crossword clue is the 8 letter word INTONING
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More clues leading to the result INTONING
Buddy explains!

The straight part
We know that "repeating mantra" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case repeating mantra = intoning.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 INTONING mean?
INTONING verb- To give tone or variety of tone to; to vocalize.
- To utter with a musical or prolonged note or tone; to speak or recite with singing voice; to chant.
- To utter a tone; utter a protracted sound.
- Intonation