Father's blade which can open doors (8)
I believe the answer is:
password
'can open doors' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'father's blade' is the wordplay.
'father's' becomes 'pa's' (pa can mean dad or father).
'blade' becomes 'sword ' (synonyms).
'pas'+'sword'='PASSWORD'
'which' is the link.
Can you help me to learn more?
(Other definitions for password that I've seen before include "Characters that gain one access?" , "Secret requirement for access" , "one admits" , "Eg, open sesame" , "parole" .)