Crossword Genius

Sign above or below a letter denoting its different phonetic value (9)

Ross

I believe the answer is:

diacritic

(Other definitions for diacritic that I've seen before include "one showing a pronounced difference" , "Mark" , "accent, for example" , "grave perhaps" , "Capable of distinguishing" .)

I've seen this clue in the Sydney Morning Herald.
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