The most common and most likely answer to this crossword clue is the 4 letter word NEUM
N
E
U
M
Buddy explains!
The straight part
We know that "group of notes sung on same note" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case group of notes sung on same note = neum.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 NEUM mean?
NEUM noun- Any of a set of signs used in early musical notation.
- A sequence of notes to be sung to one syllable.