The most common and most likely answer to this crossword clue is the 6 plus 8 letter word DOUBLE STOPPING
I've seen this crossword clue in The Guardian
D
O
U
B
L
E
S
T
O
P
P
I
N
G
Buddy explains!
The straight part
We know that "playing two notes at once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case playing two notes at once = double stopping.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 DOUBLE STOPPING mean?
DOUBLE STOPPING verb- To play on two stopped strings of a stringed instrument simultaneously.