The extra sixth, supposedly? (5)

I believe the answer is:

sense

I'm a little stuck... Click here to teach me more about this clue!

'the extra sixth' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sense that I've seen before include "reasonable" , "Pick up point" , "Meaning; perception" , "Become aware of" , "Gumption -- feeling" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.