The most common and most likely answer to this crossword clue is the 6 letter word SHARPS
I've seen this crossword clue in The Daily Telegraph
S
H
A
R
P
S
Buddy explains!

The straight part
We know that "Notes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case notes = sharps.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 SHARPS mean?
SHARPS noun- The symbol ♯, placed after the name of a note in the key signature or before a note on the staff to indicate that the note is to be played a semitone higher.
- A note that is played a semitone higher than usual; denoted by the name of the note that is followed by the symbol ♯.
- A note that is sharp in a particular key.
- The scale having a particular sharp note as its tonic.
- (usually in the plural) Something that is sharp.
- A dishonest person; a cheater.
- Part of a stream where the water runs very rapidly.
- A sewing needle with a very slender point, more pointed than a blunt or a between.
- (in the plural) Fine particles of husk mixed with coarse particle of flour of cereals; middlings.
- An expert.
- A sharpie (member of Australian gangs of the 1960s and 1970s).
- To raise the pitch of a note half a step making a natural note a sharp.
- To play tricks in bargaining; to act the sharper.
- To sharpen.