Top form (5)

I believe the answer is:

sixth

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'form' 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 can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for sixth that I've seen before include "Musical interval from A to F, say" , "Beaten by five" , "ordinal" , "fraction" .)

I've seen this clue in the LA Times and The Guardian.

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