Old man's weapon that's need to gain access (8)
I believe the answer is:
password
'need to gain access' is the definition.
I can't tell whether this defines the answer.
'old man's weapon' is the wordplay.
'old man's' becomes 'pa's' (both can mean someone's father).
'weapon' becomes 'sword' (sword is a kind of weapon).
'pas'+'sword'='PASSWORD'
'that's' is the link.
Can you help me to learn more?
(Other definitions for password that I've seen before include "Security input" , "parole" , "Shibboleth - secret phrase to gain admission" , "that'll secure one's access" , "Secret phrase needed to gain entry" .)