Warning of a sort (4)
I believe the answer is:
omen
'warning of a sort' 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 "Prognostication" , "Sign of a future event, good or bad" , "Prophetic significance" , "Warning token" , "Augury, portent" .)