Nile region once served up excellent cake (5)
I believe the answer is:
nubia
'nile region once' is the definition.
(historical region around the Nile)
'up excellent cake' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'cake' becomes 'bun' (bun is a type of cake).
'ai'+'bun'='aibun'
'aibun' reversed gives 'NUBIA'.
'served' acts as a link.
(Other definitions for nubia that I've seen before include "Ancient region of Africa" , "Area of Egypt/Sudan" , "Ancient African region absorbed into southern Egypt" , "Old African state" , "Region of northeast Africa" .)