Father Brown's ultimate temptress (5)
I believe the answer is:
siren
'temptress' is the definition.
('siren' can be a synonym of 'temptress')
'father brown's ultimate' is the wordplay.
'father' becomes 'sire' ('sire' can be a synonym of 'father').
'ultimate' says to take the final letters.
The last letter of 'brown' is 'n'.
'sire'+'n'='SIREN'
(Other definitions for siren that I've seen before include "Dangerously alluring woman" , "Loud warning device" , "Loud whistle or dangerous woman" , "Temptress; warning signal" , "Warning device - femme fatale" .)