Warning signal (5)
(Other definitions for alarm bell that I've seen before include "It rings as a warning" , "Ring it to signal danger" , "Warning sound" , "Warning device" , "Warning signal" .)
(Other definitions for siren that I've seen before include "Hooter - dangerous woman?" , "Alarm - fascinating woman" , "Warning device, alluring woman" , "'Delilah!'" , "Alarm - charming female" .)
'warning signal' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for alarm that I've seen before include "Feeling of fear" , "Device sounding danger" , "Agitate" , "State of (sudden) fear" , "Scare or worry" .)