Quick beer before restaurant emptied (5)
I believe the answer is:
alert
'quick' is the definition.
(thesaurus)
'beer before restaurant emptied' is the wordplay.
'beer' becomes 'ale' (I've seen this before).
'before' says to put letters next to each other.
'emptied' says to hollow out the word (remove centre letters).
'restaurant' with its middle taken out is 'rt'.
'ale'+'rt'='ALERT'
(Other definitions for alert that I've seen before include "'Attentive, watchful (5)'" , "Watchful, sharp" , "Danger warning" , "Warning signal" , "Signal danger" .)