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' means one lot of letters go next to another.
'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)'" .)