Sacred greeting by day (8)
I believe the answer is:
hallowed
'sacred' is the definition.
(I know that sacred can be written as hallowed)
'greeting by day' is the wordplay.
'greeting' becomes 'hallo' (alternative to hello).
'by' says to put letters next to each other.
'day' becomes 'wed' (abbreviation for Wednesday).
'hallo'+'wed'='HALLOWED'
(Other definitions for hallowed that I've seen before include "Reverenced" , "sacrosanct" , "Consecrated, holy" , "Consecrated, blessed" , "Sanctified" .)