Warning from porter perhaps emptied restaurant (5)
I believe the answer is:
alert
'warning' is the definition.
(I know that alert is a type of warning)
'porter perhaps emptied restaurant' is the wordplay.
'porter perhaps' becomes 'ale' (porter is a kind of ale).
'emptied' says to hollow out the word (remove centre letters).
'restaurant' with its centre taken out is 'rt'.
'ale'+'rt'='ALERT'
'from' is the link.
(Other definitions for alert that I've seen before include "Vigilant - warning" , "Wakeful, observant" , "Signal danger" , "Thinking and acting quickly" , "Vigilantly attentive" .)