Alarm shown by father and son? Not so (5)
I believe the answer is:
siren
'alarm' is the definition.
(siren is a kind of alarm)
'father and son? not so' is the wordplay.
'father' becomes 'sire' ('sire' can be a synonym of 'father').
'and' is a charade indicator (letters next to each other).
'not' suggests deleting specific letters.
'son' with 'so' taken out is 'n'.
'sire'+'n'='SIREN'
'shown by' acts as a link.
(Other definitions for siren that I've seen before include "Warning sound" , "Dangerously alluring woman" , "Bewitching singer,part- woman, part-bird" , "Warning device - femme fatale" , "Warning at sea" .)