The most common and most likely answer to this crossword clue is the 7 letter word BOURDON
B
O
U
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D
O
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Buddy explains!
The straight part
We know that "Organ stop" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case organ stop = bourdon.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 BOURDON mean?
BOURDON noun- The burden or bass of a melody.
- The drone pipe of a bagpipe.
- The lowest-pitched stop of an organ.
- The lowest-pitched bell of a carillon.
- A large, low-pitched bell not part of a diatonically tuned ring of bells.
- A bumblebee, genus Bombus.
- A pilgrim's staff.