Wide awake right after beer (5)
I believe the answer is:
alert
'wide awake' is the definition.
(I've seen this before)
'right after beer' is the wordplay.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'after' is a charade indicator (letters next to each other).
'beer' becomes 'ale' (type of beer).
'rt' put after 'ale' is 'ALERT'.
(Other definitions for alert that I've seen before include "Prompt" , "Wide awake and watchful" , "On the ball" , "Vigilant, watchful" , "'Attentive, watchful (5)'" .)