Father with new femme fatale (5)
I believe the answer is:
siren
'femme fatale' is the definition.
(I know that femme fatale can be written as siren)
'father with new' is the wordplay.
'father' becomes 'sire' (synonyms).
'with' says to put letters next to each other.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'sire'+'n'='SIREN'
(Other definitions for siren that I've seen before include "Temptress; warning signal" , "Attractive lady" , "Rinse (anag) - sea nymph" , "Loud whistle or sea witch" , "Loud warning signal" .)