Crossword Genius

Sounds certain to start as a schoolteacher (3)

Ross

I believe the answer is:

sir

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

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sir that I've seen before include "Way to address gentleman" , "Honourable title" , "Polite form of address to a man" , "address to teacher" , "Noble title of undesirable kind" .)

I've seen this clue in the Evening Standard.
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