Harbinger (4)
'harbinger' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for omen that I've seen before include "Prophetic significance" , "Augury, portent" , "Portent, prophetic indicator" , "Sign of something to come" , "Event regarded as a prophetic sign of good or evil" .)
'harbinger' is the definition.
(I know that harbinger can be written as herald)
This is the entire clue.
(Other definitions for herald that I've seen before include "Proclaimer" , "Official crier or messenger" , "Harbinger, messenger" , "Announce, presage" , "and 14: 'Hark! The -- -- sing' (carol)" .)