Forewarns (6)
I believe the answer is:
alerts
'forewarns' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alerts that I've seen before include "Puts on guard, forewarns" , "danger-signals" , "Puts on guard, warns about impending danger" , "Alarms, warning signals" , "Warnings" .)