Yes - right, guv'nor! (3)
I believe the answer is:
sir
'yes' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'right guv'nor' is the wordplay.
I cannot really understand how this works, but
'right' could be 'r' (common abbreviation) and 'r' is found within the answer.
The remaining letters 'si' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Feted fellow" , "Gentleman's form of address" , "Polite term of address for a man" , "Term of address for a gentleman" , "Title of knight or baronet" .)