Entirely sacred, so we hear (6)
I believe the answer is:
wholly
'entirely' is the definition.
(I know that entirely can be written as wholly)
'sacred so we hear' is the wordplay.
'sacred' becomes 'holy' (similar in meaning).
'so we hear' shows a homophone (sound like).
'holy' is a homophone of 'WHOLLY'.
(Other definitions for wholly that I've seen before include "Totally" , "in full?" , "in every respect" , "Entirely, fully" , "in every area" .)