Sheer genius absorbing new declaration (9)
I believe the answer is:
utterance
'declaration' is the definition.
(thesaurus)
'sheer genius absorbing new' is the wordplay.
'sheer' becomes 'utter' ('utter' can be a synonym of 'sheer'**).
'genius' becomes 'ace' ('ace' can be a synonym of 'genius').
'absorbing' is an insertion indicator.
'new' becomes 'n'.
'ace' going around 'n' is 'ance'.
'utter'+'ance'='UTTERANCE'
(Other definitions for utterance that I've seen before include "Something said" , "Word" , "Statement" , "sermon, say" , "Vocal expression" .)