Address of the sisters (12)
I believe the answer is:
presentation
'address of the sisters' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'address of' is the wordplay.
'address' becomes 'present' (presenting is a kind of addressing).
'of' becomes 'ation' (I can't explain this - if you can you should give a lot more credence to this answer).
'present'+'ation'='PRESENTATION'
Can you help me to learn more?
(Other definitions for presentation that I've seen before include "show" , "Display" , "Formal act of giving" , "Illustrated formal talk" , "when gifts are handed out?" .)