Vigilantly attentive (5)
I believe the answer is:
alert
'vigilantly attentive' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alert that I've seen before include "Wide awake and watchful" , "Wide-awake and watchful" , "Warning signal" , "In state of watchfulness" , "Watchful and bright" .)