Completely saintly, by the sound of it (6)
I believe the answer is:
wholly
'completely' is the definition.
(I know that completely can be written as wholly)
'saintly by the sound of it' is the wordplay.
'saintly' becomes 'holy' (both can mean religious).
'by the sound of it' shows a homophone (sound like).
'holy' is a homophone of 'WHOLLY'.
(Other definitions for wholly that I've seen before include "in every respect" , "in every area" , "Without exception" , "in full?" , "One hundred per cent" .)