Father's weapon is the countersign (8)
I believe the answer is:
password
'the countersign' is the definition.
(synonyms)
'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' acts as a link.
(Other definitions for password that I've seen before include "Characters that gain one access?" , "one admits" , "Sign to gain admittance" , "key" , "It's something to admit" .)