Fully attentive (5)
I believe the answer is:
alert
'fully attentive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alert that I've seen before include "Wakeful, observant" , "Watchfully awake" , "'Attentive, watchful (5)'" , "Watchful and bright" , "Wide awake and watchful" .)