No heat when the heat is really on (5)
I believe the answer is:
final
'no' 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?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for final that I've seen before include "Last, ultimate" , "Not to be altered" , "Concluding" , "big match" , "The last exam or leg of competition" .)