Wide-awake? Could be later! (5)
I believe the answer is:
alert
'wide-awake?' is the definition.
('alert' can be similar in meaning to 'wide-awake')
'could be later' is the wordplay.
'could be' indicates anagramming the letters.
'later' with letters rearranged gives 'ALERT'.
(Other definitions for alert that I've seen before include "Wide-awake and watchful" , "In state of watchfulness" , "'Attentive, watchful (5)'" , "Vigilant - warning" , "'Attentive, vigilant (5)'" .)