(Another definition for macron that I've seen is " French president".)
'diacritical mark' is the definition.
(I know that tilde is a type of diacritical mark)
This is the entire clue.
(Other definitions for tilde that I've seen before include "Wavy line in Spanish" , "Diacritical sign over letter" , "Squiggly mark" , "Spanish cedilla" , "Style detail" .)