Alarm shown by father and son, ultimately (5)
I believe the answer is:
siren
'alarm' is the definition.
(I know that siren is a type of alarm)
'father and son ultimately' is the wordplay.
'father' becomes 'sire' (synonyms).
'and' says to put letters next to each other.
'ultimately' suggests the final letters.
The last letter of 'son' is 'n'.
'sire'+'n'='SIREN'
'shown by' is the link.
(Other definitions for siren that I've seen before include "Seductive woman" , "Loud whistle or sea witch" , "Device producing a loud wailing sound as a signal" , "Hooter - femme fatale" , "Temptress - makes a loud noise!" .)