A happy one can be dramatic (6)
I believe the answer is:
ending
'a happy one can' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'be dramatic' is the wordplay.
'be' becomes 'end' (ending is a kind of being).
'dramatic' becomes 'ing' (I can't justify this - if you can you should give a lot more credence to this answer).
'end'+'ing'='ENDING'
Can you help me to learn more?
(Other definitions for ending that I've seen before include "Putting a stop to" , "Closure" , "Last part" , "Armageddon?" , "Finishing" .)