Crossword Genius

No heat when the heat is really on (5)

Ross

I believe the answer is:

final

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'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" .)

I've seen this clue in The Globe and Mail.
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