The most common and most likely answer to this crossword clue is the 5 plus 7 letter word RIGHT HANDERS
I've seen this crossword clue in The Daily Telegraph
R
I
G
H
T
H
A
N
D
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Buddy explains!
The straight part
We know that "Pianists ignoring the bass clef" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pianists ignoring the bass clef = right handers.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 RIGHT HANDERS mean?
RIGHT HANDERS noun- One who is right-handed.
- A slap or punch with the right hand.