On the piano, the violin part may seem impertinent! (3)
I believe the answer is:
lip
'seem impertinent' is the definition.
I can't tell whether this definition defines the answer.
'on the piano the violin part' is the wordplay.
'on' says to put letters next to each other.
'the piano' becomes 'p' (musical abbreviation).
'the violin part' becomes 'li' (I can't explain this - if you can you should give a lot more credence to this answer).
'p' after 'li' is 'LIP'.
'may' is the link.
Can you help me to learn more?
(Other definitions for lip that I've seen before include "Fleshy fold of mouth tissue" , "Sauce" , "Impudent talk from part of the face" , "Rim of an orifice" , "giving such insincere service" .)