Herald (9)
There are a few possibilities for this:
harbinger
'herald' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for omen that I've seen before include "Sign of things to come" , "Prophetic significance" , "Portent, prophetic sign" , "Straw in the wind" , "Auspice" .)
'herald' is the definition.
(synonyms)
This is all the clue.
(Other definitions for hail that I've seen before include "Greeting - falling stones" , "Frozen rain or Roman greeting" , "Acclaim; frozen rain" , "Precipitation of ice pellets" , "Weather - greeting" .)
(Other definitions for mark that I've seen before include "Gospel author" , "Identify" , "Scratch - German coin" , "Target; become stained" , "Starting point in race" .)
'herald' is the definition.
(I know that herald can be written as harbinger)
This is all the clue.
(Other definitions for harbinger that I've seen before include "Early sign" , "Herald - forerunner" , "It signals the future" , "sign of things to come" , "omen" .)