You can exercise your right to keep it (3)
I believe the answer is:
fit
'you can' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe you can see a link between them that I can't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fit that I've seen before include "Appropriate kind of convulsion" , "Physically well" , "Sudden attack,of coughing say" , "Of a suitable standard" , "Ready" .)