That's right, the old South (3)
I believe the answer is:
'that's right' is the definition.
(right can mean yes or okay)
'the old south' is the wordplay.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'south' becomes 'S' (synonyms).
(Other definitions for yes that I've seen before include "Agreed", "Affirmative reply", "Word of affirmation", "Of course", "Yea".)