Alarm father and knight (5)
I believe the answer is:
siren
'alarm' is the definition.
(siren is a kind of alarm)
'father and knight' is the wordplay.
'father' becomes 'sire' ('sire' can be a synonym of 'father').
'and' means one lot of letters go next to another.
'knight' becomes 'n' (chess).
'sire'+'n'='SIREN'
(Other definitions for siren that I've seen before include "Seductive woman" , "'Delilah!'" , "Device making a loud wailing sound" , "Temptress - makes a loud noise!" , "Alarm; temptress" .)