Crossword Genius

Sounding hard, nothing has replaced one chapter in something easy (6)

Ross

I believe the answer is:

phonic

'sounding' is the definition.
(relating to the sounds of a language)

I don't understand the remainder of the clue.

(Other definitions for phonic that I've seen before include "Chopin (anag) - related to learning by sounds" , "Of vocal sounds" , "Chopin (anag) - relating to sound" , "Involved with sound" .)

I've seen this clue in The Guardian.
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