Archaic greeting (3,4)
I believe the answer is:
all hail
'archaic greeting' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for all hail that I've seen before include "dire weather forecast" , "that may greet one" , "Appalling weather" , "Archaic greeting" , "that greeted Macbeth" .)