The most common and most likely answer to this crossword clue is the 9 letter word ECHOLALIA
I've seen this crossword clue in The Guardian
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More clues leading to the result ECHOLALIA
Buddy explains!
The straight part
We know that "Senseless repetition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case senseless repetition = echolalia.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 ECHOLALIA mean?
ECHOLALIA noun- The immediate, involuntary, and repetitive echoing of words or phrases spoken by another.
- An infant's repetitive imitation of vocal sounds spoken by another person, occurring naturally during childhood development.
- Any apparently meaningless, repetitious noises, especially voices.