Father Brown's last warning (5)
I believe the answer is:
siren
'warning' is the definition.
(I've seen this before)
'father brown's last' is the wordplay.
'father' becomes 'sire' (synonyms).
'last' 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 "Alarm - fascinating woman" , "Warning sound" , "Dangerously fascinating woman" , "Hooter - dangerous woman?" , "Bewitching singer,part- woman, part-bird" .)