Alarm mermaid? (5)
I believe the answer is:
siren
I believe this is a double definition.
'alarm' is the first definition.
(I know that siren is a type of alarm)
'mermaid?' is the second definition.
(thesaurus)
(Other definitions for siren that I've seen before include "Loud whistle or dangerous woman" , "Temptress; warning signal" , "Loud warning signal" , "Hooter - femme fatale" , "Temptress - makes a loud noise!" .)