Troubling sign (4)
I believe the answer is:
omen
'troubling sign' 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 "Foreboding" , "Prophetic significance" , "Portent of things to come" , "Prognostication" , "Sign of a future event, good or bad" .)