Warning devices (6)
'warning devices' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sirens that I've seen before include "Devices that warn with a loud wailing noise" , "Warning devices with loud wailing noise" , "Sea nymphs" , "Loud wailing warnings sounds" , "Alluring women" .)
'warning devices' is the definition.
('alarm' can be a synonym of 'warning device')
This is all the clue.
(Other definitions for alarms that I've seen before include "Warning signals" , "Fills with apprehension" , "Clocks that wake sleepers" , "Frightens, makes afraid" , "Disturbs" .)