Comet, to some (4)
I believe the answer is:
omen
'comet to some' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for omen that I've seen before include "Prophetic significance" , "Forewarning, harbinger" , "Sign of a future event, good or bad" , "Harbinger, prophetic sign" , "Warning token" .)