Sacred? Completely, by the sound of it (4)
I believe the answer is:
holy
'sacred?' is the definition.
('holy' can be similar in meaning to 'sacred')
'completely by the sound of it' is the wordplay.
'completely' becomes 'wholly' (synonyms).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'wholly' sounds like 'HOLY'.
(Other definitions for holy that I've seen before include "Sacrosanct" , "perfect, morally and spiritually" , "Dedicated to God" , "Charlemagne was the first" , "Hallowed" .)