Sounds certain to start as a schoolteacher (3)
I believe the answer is:
sir
'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" .)