Warning - femme fatale! (5)
I believe the answer is:
siren
I believe this clue is a double definition.
'warning' is the first definition.
(thesaurus)
'femme fatale' is the second definition.
(I've seen this before)
(Other definitions for siren that I've seen before include "Seductress" , "Warning sound" , "Vamp; alarm" , "Hooter - dangerous woman?" , "Bewitching singer" .)