Father Brown finally reveals femme fatale (5)
I believe the answer is:
siren
'femme fatale' is the definition.
(I've seen this before)
'father brown finally' is the wordplay.
'father' becomes 'sire' ('sire' can be a synonym of 'father').
'finally' suggests the final letters.
The final letter of 'brown' is 'n'.
'sire'+'n'='SIREN'
'reveals' acts as a link.
(Other definitions for siren that I've seen before include "Seductive woman" , "Warning at sea" , "Rinse (anag) - sea nymph" , "Alarm - fascinating woman" , "Hooter - dangerous woman?" .)