Dad's weapon is a security measure (8)
I believe the answer is:
password
'a security measure' is the definition.
(I've seen this before)
'dad's weapon' is the wordplay.
'dad's' becomes 'pa's' (both can mean one's father).
'weapon' becomes 'sword' (sword is a kind of weapon).
'pas'+'sword'='PASSWORD'
'is' is the link.
(Other definitions for password that I've seen before include "one maybe stolen by 21 [HACKER]" , "Security input" , "security measure" , "Secret requirement for access" , "Eg, open sesame" .)