Sign to heed (4)
I believe the answer is:
omen
'sign to heed' 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 "Harbinger, prophetic sign" , "Foreboding" , "Portent of evil" , "Forecast" , "Forewarning, harbinger" .)