Taking it all in (5)
I believe the answer is:
alert
'taking it all in' 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 "Watchful; warning" , "Watchfully awake" , "Warning signal" , "Vigilant, watchful" , "'Attentive, vigilant (5)'" .)