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" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">c for one</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that C is a type of keynote)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)