Alter in the case of quiet mimic (7)
I believe the answer is:
reshape
'alter' is the definition.
(thesaurus)
'in the case of quiet mimic' is the wordplay.
'in the case of' becomes 're' (short for 'regarding' ie relating to, about).
'quiet' becomes 'sh'.
'mimic' becomes 'ape' (I've seen this before).
're'+'sh'+'ape'='RESHAPE'
(Other definitions for reshape that I've seen before include "Change the form of" , "Give different form to" .)