Identical to the southern part of America (4)
I believe the answer is:
same
'identical' is the definition.
(similar in meaning)
'to the southern part of america' is the wordplay.
'to the southern' becomes 's' (abbreviation).
'part of america' becomes 'ame' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
's'+'ame'='SAME'
Can you help me to learn more?
(Other definitions for same that I've seen before include "Identical, unchanged" , "Aforesaid" , "Equal in amount or value" , "Identical, not different" , "If - be the food of love, play on (Twelfth Night)" .)