Agreeable thing to say? Emphatically! (3,3)

I believe the answer is:
yes sir
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for yes sir that I've seen before include "Aye aye Boss!" , "aye-aye!" .)
I've seen this clue in the Evening Standard.
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