Warns (6)
I believe the answer is:
alerts
'warns' is the definition.
(alerting is a kind of warning)
This is the entire clue.
(Other definitions for alerts that I've seen before include "Alarms, warning signals" , "danger-signals" , "Draws attention to, a danger perhaps" , "Puts on guard, forewarns" , "Warns, draws attention to" .)