Father's weapon is a security measure (8)
I believe the answer is:
password
'security measure' is the definition.
(I've seen this before)
'father's weapon' is the wordplay.
'father's' becomes 'pa's' (pa can mean dad or father).
'weapon' becomes 'sword' (sword is a kind of weapon).
'pas'+'sword'='PASSWORD'
'is a' is the link.
(Other definitions for password that I've seen before include "Requirement for logging in" , "It's something to admit" , "Eg, open sesame" , "Shibboleth - secret phrase to gain admission" , "Security input" .)