Substitute mixed cereal's eaten quietly (7)
I believe the answer is:
replace
'substitute' is the definition.
(synonyms)
'mixed cereal's eaten quietly' is the wordplay.
'mixed' indicates anagramming the letters.
'eaten' indicates putting letters inside (some letters have eaten others).
'quietly' becomes 'p' (p for piano and quiet).
'cereal' with letters rearranged gives 'relace'.
'relace' placed around 'p' is 'REPLACE'.
(Other definitions for replace that I've seen before include "Put back in original position" , "Put back or substitute" , "Put something back where it belongs" , "Substitute for (someone)" , "Put back where you found it" .)