Warn one to be vigilant (5)
I believe the answer is:
alert
I believe this clue is a double definition.
'warn' is the first definition.
(I know that alert is a more specific form of the action the warn)
'one to be vigilant' is the second definition.
(similar in meaning)
(Other definitions for alert that I've seen before include "Watchful; warning" , "Put on guard, warn" , "Signal danger" , "Quick to notice" , "Wide awake and watchful" .)