Old man's weapon that's needed to deal with challenge (8)
I believe the answer is:
password
'deal with challenge' is the definition.
I can't judge whether this definition defines the answer.
'old man's weapon that's needed' is the wordplay.
'old man's' becomes 'pa's' (both can mean someone's father).
'weapon' becomes 'sword' (sword is a kind of weapon).
'that's needed' is a charade indicator (letters next to each other).
'pas'+'sword'='PASSWORD'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for password that I've seen before include "Characters that gain one access?" , "that'll secure one's access" , "parole" , "Secret requirement for access" , "ID (to access computer)" .)