Warning signal after alcoholic drink? Right (5)
I believe the answer is:
alert
'warning signal' is the definition.
(synonyms)
'after alcoholic drink? right' is the wordplay.
'after' means one lot of letters go next to another.
'alcoholic drink?' becomes 'ale' (type of beer).
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'ale' next to 'rt' is 'ALERT'.
(Other definitions for alert that I've seen before include "Wakeful, observant" , "Quick to notice" , "Wide-awake and watchful" , "In state of watchfulness" , "Vigilantly attentive" .)