Fully caffeinated, say (5)
I believe the answer is:
alert
'fully caffeinated say' 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 "Warning signal" , "Wide awake and watchful" , "Watchful; warning" , "Watchful and observant" , "Vigilantly attentive" .)