What the chap dropped in the cap (5)

I believe the answer is:

aitch

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

'cap' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for aitch that I've seen before include "first bit of hymn?" , "Eighth letter of the English alphabet" , "the start of hives?" , "that I will immediately succeed" , "bone" .)

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.