C, for one
(Other definitions for arc that I've seen before include "Semi-circle" , "Portion of a circle" , "circle, partly" , "Luminous electrical discharge" , "Curving trajectory" .)
'c for one' is the definition.
(I know that C is a type of keynote)
This is the entire clue.
(Other definitions for note that I've seen before include "A, B, C, D, F or G" , "Heed; short letter" , "Write down" , "Single tone; short letter" , "Take this to heed" .)