Sign of something to come (7)
(Other definitions for portent that I've seen before include "Sign or warning of an event likely to happen" , "Augury" , "Sign, omen" , "Foreboding" , "Warning of momentous event" .)
'sign of something to come' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for omen that I've seen before include "Gloomy sign" , "Portent, prophetic indicator" , "Augur" , "Portent of evil" , "Augury, portent" .)