That's right, the old South (3)
I believe the answer is:
yes
'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).
'ye'+'s'='YES'
(Other definitions for yes that I've seen before include "3 across" , "Validating vote" , "OK" , "Affirmative answer" , "Agreed" .)