Warning device (5)
'warning device' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for siren that I've seen before include "Hooter - femme fatale" , "Loud whistle or kind of witch" , "Loud whistle for sea temptress" , "Nymph supposed to have lured sailors to destruction on rocks" , "Alarm; temptress" .)
'warning device' is the definition.
(synonyms)
This is all the clue.
(Other definitions for alarm that I've seen before include "Distress signal" , "Warning sound or signal" , "Anxious awareness of danger" , "Warning sound/device" , "uneasiness" .)
'warning device' is the definition.
(fire alarm is a kind of warning device)
This is all the clue.
(Other definitions for fire alarm that I've seen before include "Warning of conflagration" , "Warning sound" , "Safety device" , "It warns of smoke and flames" , "Warner" .)